On th Maxillare y Gland an somds e other Features in the ...jcs.biologists.org/content/joces/s2-59/235/401.full.pdf · NOTE OSN SOME OTHER FEATURE OP TH INTERNAE S L ANATOMY ... in

INTERNAL ANATOMY OF SQUILLA. 401

On the Maxillary Glands and some otherFeatures in the Internal Anatomy of

Squilla.

By

W. W. F. Woodland,

Tlie Muir Central College, Allahabad, India

(late of University College, London).

With Plate 28 and 9 Text-figures.

CONTENTS.PAGE

I. T H E MAXILLAKY GLANDS . . . . 401

Preliminary Comments . . . . 401

The Gross Structure of the fully formed Gland . . 403

The Histology and Cytology of the Gland . . 404Some Stages in the Development of the Gland . . 416

Remarks on the Maxillary Gland of Squilla compared withthe Excretory Leg Glands of other Crustacea and

Arthropoda . . . . . 420II . NOTES ON SOME OTHER FEATURES OP THE INTERNAL

ANATOMY . . . . . 423

The Communications between the Pyloric Digestive Glandand the Gut . . . . . 4 2 3

The Rectal Glands . . . . . 4 2 5The Nauplius Eye . . . . . 4 2 6

A P P E N D I X : TECHNIQUE . . . . 426

I. T H E MAXILLARY GLANDS.

P re l imina ry Comments.

THE following description of the maxillary gland of Squilla

was intended to form but a small part of a monograph dealing

VOL. 59 , PABT 3 . NEW SERIES. 26

402 ^Y. N. F. WOODLAND.

with the anatomy in general of this aberrant Crustacean, butin consequence of my appointment to the Chair of Zoology atthe Muir Central College, Allahabad, this project of a mono-graph has had to be abandoned. Since the description, sofar as it goes, is fairly complete and intelligible, I havethought it worth while to publish it, together with a fewnotes on other parts of the anatomy of Squilla.

As is well known, the general text-book statement thatantennal glands are characteristic of adult Malacostraca inthe same way that maxillary glands are of " Entomostraca "now admits of many exceptions, since maxillary glands havebeen found in a number of undoubted Malacostracan genera.Maxillary glands have been described in adult forms ofLeptostraca (Nebalia, Claus [7]), of Syncarida (Anaspides[3]), of Tanaidacea (Apseudes, Claus [8]), of Isopoda(Bopyrus, Gyge, Porce l l io , L ig ia , Asel lus , Rogen-hofer[21]), and of Stomatopoda (Squilla, Kowalevsky [14]).Since the researches of Grobben (10), Marchal (17), Kingsley(13), Waite (23), and others have demonstrated the highdegree of complexity attained by the antennal glands in thehigher and.larger Malacostraca, it is perhaps surprising thatthe condition of the glands in the large-si zed and, in manyrespects, highly developed and aberrant Squillidas should nothitherto have attracted the attention of zoologists, especiallyin view of the statement of Kowalevsky (14) that the glandsare maxillary and not antennal. However, apart from therough figure by Claus (4, Taf. iv, fig. 8) of the maxillaryelands in a late Stomatopod larva, the bald statement ofKowalevsky that they exist in the adult Squilla, and the pre-liminary description of these glands which I read (25) beforeSection D at the 1911 meeting of the British Association forthe Advancement of Science (Portsmouth), nothing has beenpublished on the subject.

As material for this inquiry, I employed adult specimensof Squilla desmarest i i (Risso) and Erichthus larvae ofLysiosqui l la eusebia (2 mm. in length, aud comparablewith the figures 191 A and B shown in Caiman [3]), obtained

INTERNAL ANATOMY 0*' SQUIXLA. 403

from Naples and fixed and preserved by the methodsenumerated in the Appendix ; also well-fixed Erichthus larveeof an unknown species of Squilla, 12 mm. and 14 mm. inlength respectively, preserved in the store-room at UniversityCollege, Londou. Several complete series of transverse andlongitudinal vertical sections were made of both adult andlurval specimens, and the following description is based uponthe careful study of these sections, assisted by dissectionsboth of adult Squi l la desmares t i i and Squi l la mant isand whole mounts of the smaller larvas.

Before proceeding further I wish to offer my sincere thanksto Dr. W. T. Caiman for kiudly seeing the MS. through thepress, and to Miss B. M. Brown for the careful drawingswhich illustrate the paper.

THE MAXILLAKY GLANDS OF THE ADULT SQUILLA DESMARESTII.

Macroscopic Appea rance .

The paired maxillary glands lie at the hinder end of the" neck " of the animal, nearly on a level with the proximaljoint and epipodite of the large subchelate second thoraciclimb. Previous to dissection they may be seen through thetransparent cuticle covering the sides of the neck as largeyellowish masses, provided that the anterior thoracic limbsbe pulled outwards from their forwardly directed positionunder the sides of the carapace. On removal of the dorsalcarapace the paired glands are found to lie just under thehypoderinis. Superficially, the glands closely resemble a pairof adductor muscles (the vertically descending gland-ductssimulating tendons) connected with the mandibles (PI. 28,fig. 1), and it seems probable that they have been identifiedas such by previous observers, since otherwise it is difficultto understand how such conspicuous structures could escapeattention. Viewed from the dorsal aspect, each of the twoglands is seen to be a pear-shaped, compact yellowish mass,which tapers ventrally into a thin stalk, the stalks of the twoglands converging towards the median line and disappearing

404 W. N. F. WOODLAND.

one on each side of the large sub-oesophageal compoundganglion {U. Th. G., PI. 28, fig. 1). On closer inspection (PI.28, fig. 2) the thick upper portion of the pear-shaped gland isseen to be deeply grooved on its outer side to allow space fora large body-muscle which happens to traverse the body-cavity in the proximity of the gland. Although the gland is,in parts, closely wrapped round the muscle, yet there is, ofcourse, no organic connection between the two structuresIn fig. 2 (PI. 28) the duct of the gland is seen to enter thebase of the maxilla (the second maxilla), and, after dilatingslightly to form a bladder, to open on the extremity of thesmall papilla already described by Caiman (3), who had pre-viously suggested that the minute orifice present might proveto be that of the maxillary gland. In fig. 2 (PI. 28) are alsoshown the numerous minute flask-shaped multicellular glandswhich are to be found scattered in many other regions underthe hypodermis. These glands are connected with the seta?borne on the cuticle, and have been previously figured anddescribed (under the general name of " driisenzellen," andoften incorrectly stated to be unicellular glands) by Claus (5),Jurich (12) and others. A drawing of a section through oneof these glands (showing the gland lumen) is shown in Text-fig. 8. Fig. 2 (PI. 28) represents the isolated maxilla and theattached gland, as seen in spirit under the dissecting micro-scope ; when in balsam the numerous seta glands are usuallyalmost invisible, since staining reagents rarely penetratesufficiently in order to render them conspicuous. The minutepapilla with its gland opening is situated on the convex sideof the maxilla, i. e. away from the mouth. The gland itselfis, as we shall see, closely invested with squamous epithelium,and in size is about 4 mm. long, excluding the duct and thebladder, and about 2'5 mm. in maximum width.

General His to logy.

The general position in the body and histological construc-tion of the gland may be gathered from a perusal of fig. 3

INTERNAL ANATOMY OP SQUILLA. 405

(PL 28), which represents the gland and other organs in atransverse section of the left side of this region of the body.Most of the hasmocoele in this region is seen to be occupied

TEXT-FIG. 1.

Hyp

G . D . - -

M.Mx.

Mus.

-••-Str . B.

HypTh2

Maxillary gland in a sagittal section of the larva ofLysiosquil laeusebia, 2 rum. long. End. S. Future end-sac. M.Mx.Gl.Fixture kidney. Op. Opening of gland duct. Str. B. Striatedborder of gland-cells of kidney portion (absent in cells ofend-sac). ( x 384.) For other reference letters see p. 429.

by large oblique longitudinal and transverse muscle bandsand the maxillary glands. The parts of the gland shown inthis figure, however, do not all occur in any one of my thinsections, but extend through several—the figure may be

406 W. N. E. WOODLAND.

described as piano-stereoscopic. The mass of the gland liesbehind the opening of the duct on the maxilla, seventeensections (each 10 fi thick), intervening between that exhibit-ing this opening and that showing the end-sac in the upper partof the gland. In one respect only is the part of fig. 3 (PI. 28)representing the gland obviously conventional, and that is thescale of magnification of the details showing the generalstructure of the gland mass—the cell-layers and ducts are

TEXT-FIG. 2.

Anterior

Posterior

Com.

A. Reconstructed figure of the external appearance of the glandviewed dorsally in a larva 12 mm. long. In B the end-sac issupposed to be dragged apart from the kidney in order to showthe postero-dorsal situation of the connection between thetwo. End. S. End-sac. M. Mx. Gl. Kidney portion of gland.B. Mx. Gl. Duct of gland. Op. Opening of duct. Coin. Tubularconnection between end-sac and kidney. ( X 100.)

exhibited as large and few in number, whereas in actualitythey are so small and numerous as to be undepictable in a draw-ing of this magnification. This excusable convention has theadvantage of showing at a glance the general architecture ofthe entire organ.

The essential structure of the gland will be best compre-hended by a preliminary reference to the early stages of itsdevelopment. In the earliest stage of development found in

INTERNAL ANATOMY OP SQU1LLA. 407

our material the entire gland consists of a simple deeppocket, somewhat dilated at its blind extremity, which hasapparently been formed by an involution of the hypodermis,i.e. the ectodem (Text-fig. 1). At a later stage the glandconsists of two wide sacs which only communicate with eachother by a narrow tube—the distal or " end-sac" and the

TEXT-FIG. 3.

Dorsad

Hyp..

Er.d.S.C

Cut."'

Hyp.

B.S.Ventral

Transverse section across the end-sac and terminal part of thekidney of the gland (in a transverse section of a 14 mm. larva)showing the connection (Com.) between the two. Cut. Cuticle.Hyp. Hypodermis. Sq.jEp. Squamous epithelium underlyinghypodermis. End. S. C. End-sac cells. B. S. Blood-sinus (con-tained "blood not shown). P. 0. Protoplasmic processes fromend-sac cells extending across Mood-space to squamous epi-thelium. I. M. Invagination of kidney wall in process offormation. I. P. Similar invagination of septum separatingend-sac from kidney. ( X 384.) Tor other reference letterssee p. 429.

proximal sac or gland proper or kidney, as I shall term it infuture, which opens to the exterior via the duct (Text-figs. 2and 3). During subsequent development each of thesedivisions, but especially the kidney, enlarges greatly, and


TEXT-FIG. 4.

Lum -

B.S.---££

E x c . - -

M.C.'CD".

BS--I.C.D.-

r_.D.Mx.GI.

Structure and histology of kidney and proximal duct portion ofthe gland in the adult Squilla. Lum. Lumen of gland containingexcretory matter (Exc). B. S. Blood-sinus (morphologicallyexternal to gland). 0. Sq. Ep. Outer layer of squamous epi-thelium forming external wall of blood-sinus. I.Sq.Ep. Innerlayer of squamous epithelium (forming inner wall of blood-sinus). D. Sq. Ep. The double layer of squamous epithelium.I. C. D. Invagination of duct-cells. Note absolute distinctionbetween duct (C. B.) and kidney cells (M. C.) with striatedborder (Str. S.)—intennediate types do not exist. ( x 296.)


simultaneously their lumina become invaded by numerousinvaginations of their walls (two incipient invaginations areshown in Text-fig. 3). These iuvaginations are formed at allpoints of the walls of the enlarged kidney and end-sac andbecome so long, branched and closely compacted internallyin the former that the originally spacious lumen is reduced inall parts to thin cracks lying between adjacent cell-layers(Text-fig. 4). The entire exterior of the gland is investedwith two layers of squamous epithelium, between which liesa division of the hseinocoele. The inner layer (I. Sq. Ep.,Test-fig. 4) only is closely applied to the surface of the entiregland, and becomes involved in all the invaginations justreferred to. Careful inspection of well-stained sections shows(Text-fig. 4), as might be inferred from the development ofthe gland just outlined, that the mass of the gland exhibitstwo kinds of spaces : (i) Spaces hEemocoelic in nature, andtherefore morphologically outside the gland, lined by thesquamous epithelium, which originally covered the glandsurface and which subsequently followed the invaginations ofthe gland wall ; and (ii) spaces devoid of a squamous epithe-lium, being solely bordered by the gland-cells, and represent-ing all that is left of the originally wide lumen of the gland.Thus, it will be seen that the narrow blood-space surroundingthe gland penetrates by means of the invaginations describedinto the innermost parts of the mass of the organ, and inevery part is only separated from the lumen of the gland by athin squamous epithelium and the single layer of gland-cellswhich purify the blood. The squamous epithelium formingthe outer wall (0. Sq. Ep.) of the hasmocoele, and lyingwholly on the periphery of the gland, lies for the most par t inclose contact with the innei" squamous layer (I. Sq. Ep.), wherethis is situated at the periphery, and in my sections can onlybe distinguished from it at those points where the inner layerbecomes invaginated to line the involutions of the gland wall,and so necessarily becomes separated (Text-fig. 4).

In longitudinal sections of the gland aud duct (PI. 28, fig.3), the three regions characteristic of Crustacean antenna!

410 W. N. 1'. WOODLAND.

and maxillary glands can be distinguished : (a) The region ofthe end-sac, (b) the region of the kidney, and (c) the regionof the duct.

Histology of the End-sac.

The region of the end-sac is distinguishable from the regionof the gland proper by its relatively small size as comparedwith the kidney (though early in development they are equal

TEXT-FIG. 5.

Cells of end-sac (End. S. C.) showing vacuoles (V.) and slightly-granulated cells of kidney (M. C.) next end-sac (from longi-tudinal vertical section of adult Squilla). JB. S. Very narrowblood-sinus. ( x 810.) For other reference letters see p. 429.

in size), by its spacious lumen (the invaginations of the end-sac wall being few in number and simple in character) andby the character of the cells. The position of the narrowcommunication between the lumen of the end-sac and thekidney is very difficult to detect, since the opening is appa-rently not made evident by the presence of a couple or moreof large " trichterzellen," such as those described by Allen(1) iu the maxillary gland of the larva of Paltemonetes andby Vejdovsky (22) in the antennal gland of Gammarus, andit is thus almost impossible to distinguish this narrow channelfrom the numerous other narrow channels penetrating the


entire mass of the gland. If Text-fig. 3 be referred to, it willbe seen that in the early stage of development figured, theseptum (i. e. the apposed walls) separating the end-sac fromthe kidney is giviug rise to an iuvagination in the same wayas the other parts of the gland-wall, and hence the boundarybetween these two parts of the gland becomes very sinuousin the fully formed organ (PJ. 28, fig. 3), though easily dis-tinguishable on account of the characteristic features of theend-sac cells. The end-sac as a whole is situated on theupper and slightly posterior and external lateral aspect of thefully formed gland, i . e . it retains the position in which itarose. The constituent cells of the end-sac of the fullyformed gland (Text-fig. 5) are somewhat irregular in outlineand consist of faintly granular cytoplasm, which is difficult tostain, and large nuclei. In close apposition to the nucleusof each cell is a large and very characteristic vacuole (F) ,usually much larger than the nucleus. These cells of the end-sac differ markedly from those of the gland proper, and thetwo kinds of cells are quite distinct—there are no cellstransitional in character. The cells of the fully formed end-sac also differ greatly from those of the larval end-sac, as willbe seen below. The blood-sinus, which extends into theseptum separating the end-sac from the gland proper, is soattenuated in the gland of the adult as to be only recognis-able in position by the nuclei of the two opposed layers ofsquamous epithelium composing its walls.

His to logy of the Kidney or Gland P rope r .

As already stated, in the region of the kidney, the widelumen of the original sac is reduced to slit-like spacessituated between the crowded invaginations of the wallalready described (Text-fig. 4). These thiu spaces represent-ing the gland lumen can always be distinguished from theequally thin extensions of the external hEetnocoele by thefacts that (A) the excretory matter contained in the lumen islargely in the form of spherical bubble-like masses (the blood,

412 W. X. V. WOODLAND.

on the other hand, appearing as a fine sediment or homo-

geneous film), and (B) that they are lined, not by squamous

epithelium, but by the excretoi-y cells, the border of which,

situated next the lumen, is characteristically striped. The

TEXT-FIG. 6.

...C.Str.

B.S:

C m . " "

Sq.Ep-Cell3 of middle portion of kidney showing least amount of

granulation, also striated border (Str. B.) and striations incytoplasm (0- Str.). Blood-sinus (B. S.) relatively spacious.( X 810.)

TEXT-PIG. 7.

. . .S t r .B .

LumB.S.

—C

-I.Sq.Ep" " -O .SqEp

Cells of kidney nearest duct showing greater granulation (Grn.).( X 810.) For other reference letters see p. 429.

cells composing the kidney vary slightly in character accord-ing to their position, those situated towards the end-sac, i . e .in the dorsal region, being far less granular and somewhatmore columnar in form than the cells situated nearer thegland-duct. The more dorsally situated cells (Text-fig. 6)

INTERNAL ANATOMY OP SQUJLLA. 413

are large and columnar in form. The cytoplasm containsnumerous small granules, is somewhat denser in the regionnext the hasmococele, and shows a faint striation parallel withthe cell length. The portion of the cytoplasm bordering thelumen is, as above stated, characteristically striped. Thenuclei are always situated near the striated border, and arelarge, with conspicuous chromatin granules. These lessgranular columnar dorsal cells pass gradually into the moregranular and often flatter cells found towards the duct end ofthe gland (Text-fig. 7). The ventral cells chiefly differ fromthe dorsal in that the granules contained in the cytoplasmare more numerous and are much lai-ger and that there areno striations in the general cytoplasm. I t should also bementioned that the cells composing the par t of the kidneyactually in contact with the end-sac are flat and small (Text-fig. 5), and that these gradually merge into the columnarkidney-cells already described.

His to logy of the Duct.

The duct region of the gland is distinguishable as a wholefrom the region of the kidney by the spacious lumen (a fewwall invaginatious only being present in the upper part of theduct) and by the character of the cells composing its wall.The character of the cells of the duct, indeed, enables thisregion to be distinguished with the greatest precision fromthe adjoining region of the gland proper, since, as can be seenin Text-fig. 4, there exist no cells intermediate in characterbetween those composing the duct, on the one hand, and thosecomposing the kidney on the other. We saw that the regionof the gland proper could be sharply distinguished by thesame means from the region of the end-sac, the cells of thelatter being quite distinct in character.

The duct region of the gland can (PI. 28, fig. 3) be sub-divided into three sub-regions : (1) The upper duct, (2) thebladder, and (3) the duct aperture to the exterior. In thesubregion of the upper dnct the lumen is wide, contains a

414 W. N. I1'. WOODLAND.

large quantity oC excretory products, and is only slightlyinvaded by a relatively few simple wall-involutions (PI. 28,fig. 3). The character of the cells forming the duct wall isshown in Text-fig. 4. The cytoplasm is faintly stained on theaide situated next the lumen, but denser in appearance nextthe hsemoccele and is throughout faintly granular. Thenuclei are large and spherical. In shape the cells are veryirregular, and are quite devoid of the conspicuous striatedborder so characteristic of the cells of the gland proper. Thenuclei, like those of the gland-cells, are situated at one endof the cell, but unlike those of the kidney, are situated nextthe liEemocoele and not next the lumen. Text-fig. 4 shows wellthe respective characters of the duct- and gland-cells and theabrupt transition from the one type to the other. Judgingfrom the character of the cells of the duct, it seems probablethat they take little or no share in the process of excretion.In the sub-region of the bladder the lumen is more spaciousthan in any other part of the gland and invaginations of thewall are quite absent. The cells of the walls are short,columnar or cubical in form (Text-fig. 8), and the cytoplasmis uniformly pale and faintly granular; there is no polardifferentiation as in the cells of the upper duct. The nucleiare situated at the ends of the cells remote from the lumen.It is perhaps important to notice that the walls of the bladdernear the external opening (Text-fig. 8) are devoid of thedouble layer of investing squamous epithelium found on theexterior of all other partsof the gland—a fact possibly of signifi-cance in connection with the question as to the germ-layerorigin of the gland to be discussed later.

The bladder rapidly becomes narrow at its lower end toopen to the exterior (Text-fig. 8). A notable feature of thisexternal opening is the modification of the cells lining theinner and posterior side of the duct. As will be seen fromfig. 3 (PI. 28) and Text-fig. 8, the cells in the position statedbecome enormously elongated in a direction approximatelyparallel with the length of the duct to form a conspicuousclump of columnar cells continuous at the upper end with the

INTERNAL ANATOMY OP SQUILLA. 415

cells of the bladder-wall and at the lower end with the cells ofthe hypodermis (the hypodermis has shrunk away from thecuticle all round the opening of the duct to the exterior). Thisclump of cells thus does not appear to form an ordinarysphincter musclej since the muscle-fibres are not disposed

TEXT-FIG. 8.

W.BI

Gl.

Cut.'

P.Col. Op.

C.P.Col.

Opening of the gland. SI. Lower portion of bladder. Op.Opening of du ct. P. Col. Patch of columnar epithelium at ductopening, the lower cells of which are, in life, connected {G.P. Col.)with the two curious plug-like todies situated in the cuticle(Cui.). Gl. Transverse section across one of the numeroussmall nnuticellular seta glands (driisenzellen). The hypodermishas shrunk away from the cuticle. ( X 384). For otherreference letters see p. 429.

circularly round the narrow duct and they are only developedin the inner wall. If this clump of cells be examined carefullyit is seen that the outer ends of most of these cells (containingdenser cytoplasm) are in close contact with a thick strip ofcuticle which extends up the inner wall of the duct andnowhere else, and that those cells situated at the extreme end


of this duct opening (which have presumably shrunk awayfrom the thick cuticle) are also connected with the cuticle byprotoplasmic processes (nearly a dozen in number, though onlyabout six are shown in the drawing) developed from theirouter ends, which converge towards and become inserted intotwo curious plug-like bodies firmly imbedded in the cuticle(C. P. Col., Text-fig. 8). These two plug-like bodies are notfound in any other part of the body, and doubtless possess somespecial retentive function in connection with the cell-clumphere developed. At their inner ends the cells composing theclump are ill-defined, and appear to merge into the numerousconnective-tissue strands (Conn., Text-fig. 8) occupying thecavity of the maxilla in this region. It is possible that thecells of the clump represent smooth muscle-fibres (they arenot striped transversely), which on contraction serve to blockup the external aperture of the gland. It is certain that theclump is not glandular, developed for the purpose of secretingthe extra thick cuticle in this region, since the ordinary hypo-dermis is not specially developed elsewhere to produce thisresult, and it is difficult to suppose that the clump possesses asensory function, though the pigment (?) spots associated withthe two "p lugs" at first suggest this.

Some S t a g e s in the Deve lopment of the Max i l l a ryGland.

The Grland of t he 2 mm. L a r v a of Lys io squ i l l aeusebia . — In some Erichthus larvae of Lys iosqu i l l aeusebia , 2 mm. in total length, comparable with the develop-mental stage figured, after Claus, by Caiman (3) in his fig.191A (only the second thoracic limb in my specimens has notyet become enlarged), and, with the exception of the " meta-nauplius" stage of Lister (16), representing the youngestlarval form of Squilla yet known, the maxillary gland is of thesimple form represented in Text-fig. 1. The gland at thisearly stage of development solely consists of a simple, deep,pocket-like ingrowth opening at the base of the maxilla on


its posterior outer side and possessing a blind extremity whichis somewhat dilated. This blind extremity lies just underthe junction of the free carapace [F. C.) with the body andjust behind the diverticulum of the gut (G. D.) which willsubsequently form the " liver." Text-fig. 1 is piano-stereo-scopic (combines in one plane parts which are only to befound in separate sections and which would only be visiblecollectively under the microscope if the section were ofsufficient thickness to contain them all), and represents thegland as seen in sagittal sections of the larva. In a series oftransverse sections of the larva, the gland was only includedin eight successive sections, each 10 ft thick, and appeared asa small tube cut transversely owing to the gland slantingposteriorly in position. The gland is, in all probability,functional at this early stage of development, since, with theexception of those situated at the extreme end of the diverti-culum and at the external opening, all the cells (in themiddle part of the gland labelled M. Mx. Gl.) possess thestriated border so characteristic of the excretory cells of thegland proper in the fully formed organ and contain granulesin the cytoplasm. The fact (perhaps I should say probability,since the sections do not afford quite conclusive evidence)that the extreme cells of the terminal dilatation do not possessa striated border certainly favours the conclusion that thisregion becomes the end-sac of the fully formed gland. Ifthis last statement be true (and there is but little doubtabout it), then, in view of the obvious continuity of the wallof the gland diverticulum with the hypodermis at theex.ternulopening (see Text-figs. 1 and 8) and the absence of mesen-chymatous cells in the vicinity of the gland (the gland isapparently not invested by sqnamous epithelium at thisstage), it seems extremely probable that the whole of thediverticulum (and therefore the whole of the fully formedgland, end-sac and all) is the product of an involution of thehypodermis, i . e . ectoderm. The probability of this conjec-ture is discussed below.

The G l a n d in a 12 mm . L a r v a , S q u i l l a sp.—The stage

VOL. 59 , PART 3 . NEW SERIES. 27


of development of the maxillary gland in this larva is indi-cated in Text-fig. 2, A and B, which represents a reconstructedstereoscopic view of the gland based on a series of drawings,drawn to scale, taken from the serial sections. In thisfigure the gland is viewed from the dorsal aspect (cf. PI. 28,fig. 1, showing the adult gland). In A the three parts of thegland, which are now easily distinguishable, are shown intheir correct relative positions. The end-sac (End S.) isshown to lie on the postero-dorsal aspect of, and in closeapposition with, the kidney (M. Mx. Gl.), and the latter is, ofcourse, continuous with the vertically descending duct (D. Mx.Gl.), the terminal part of which is not yet dilated to form abladder. In B the end-sac is supposed to have been pulledapart from the kiduey in order to show the posterior positionof the narrow canal (Com.) putting the two parts into com-munication.

The Gland in a 14 mm. Larva , Squ i l l a s p . —Though the Erichthus larva of this size is not much largerthan that of the preceding stage, and the maxillary glands ofboth stages are practically identical, yet the gland of the olderlarva differs in one small though important feature from thatof the younger. In a transverse section of the gland at thisstage of development (Text-fig. 3) passing through the narrowcommunication (Com.) between the end-sac and the kidney,the cavities and walls of these two regions are well shown.The cells composing the wall of the cavity representing thekidney (Me.) are very similar to those found in the gland ofthe adult Squilla. They are regular in outline and staindeeply ; the cytoplasm is granular, and there is a well-definedstriated "border present (Str. B.). The quite different cellscomposing the wall of the end-sac (.End. *b*. C.) are, on theother haDd, very dissimilar to those of the fully formed gland.As shown in the figure the faintly grannlar cytoplasm isdrawn out at irregular intervals into tent-like projections(P. C) , which stretch across the intervening space (htemocoele)to the layer of squamous epithelium surrounding the wholegland. These projections are apparently found in connection


with the end-sacs oE most autennal and maxillary glands,since they are figured by G-robben (10), Claus (6),Vejdovsky (22), and many other authors. Finally, it will benoticed that these end-sac cells contain none of the vacuolesso characteristic of the cells composing the fully formed end-sac.

As regards the squamous epithelial investment of the glandat this stage of development it will be remembered that inthe adult Squilla the whole of the gland, excepting the lowerpart of the bladder, is enclosed in a double layer ofsquamous epithelium, the two layers enclosing a blood-space.On the other hand, in the early stage of development of thegland depicted in Text-fig. 1 no epithelial investment what-ever can be observed. Text-fig. 3 shows what may perhapsbe regarded as an intermediate condition. As will be seen, thegland at this stage lies in close contact with the side of thebody, and is only separated from the hypodermis by a(apparently) single layer of squamous epithelium (8q. JEp.),which is also to be found on the inner side of the gland, andpenetrates between the closely apposed walls of the end-sacand the gland proper. This single layer of squamons epithe-lium also penetrates into the invaginatious of the wall, thepresence of which distinguishes the gland of the 14 mm. larvafrom that of the 12 mm. In this 14 mm. larva, therefore, wecan observe the commencement of the formation of thoseinvolutions of the wall of the gland proper (t.wo are shown inText-fig. 3—I.M. and I.P.), which ultimately so invade thecavity of this region as to reduce it to the winding system ofnarrow cracks already described as characteristic of the fullyformed organ. Since the single layer of investing squamousepithelium is drawn into each of these involutions of the wall,it must therefore represent the inner layer of the two layerscharacteristic of the adult condition. The outer layer ofsquamous cells of the adult gland is possibly produced later bythe splitting (delamination) of the inner (I admit that theonly evidence I have for this suggestion is the occasionalappearance, in sections, of a double condition to be found

420 W. N. P. WOODLAND.

over small areas of the single squamous layer.) The ductto the exterior is composed of cells similar to those of thegland proper, save that they lack the striated border. Thereis as yet no dilatation to form a bladder.

Remarks on the Maxi l l a ry Gland of Squ i l l a com-pared with the E x c r e t o r y Leg Glands of o therCrustacea and Ar th ropoda .

If we compare the maxillary glauds of Squilla with thoseof the Isopoda (described by Rogenhofer [21]) it is at onceevideut that the former greatly exceed the latter in com-plexity of structure, and in fact the maxillary glauds of theadult Bopyrus, Porcellio, and even Ligia are almost exactlycomparable, both as regards general form and histology, withthe early stage of development of the maxillary gland foundin the 12 mm. larva of Squilla. In Asellus aquat icus , onthe other hand, the kidney region of the gland has, accordingto Rogenhofer, become much lengthened and convoluted andtherefore more similar to the maxillary gland of Branchipus(Claus[6]) or the antennal glands of Gammarus (Vejdovsky [22]and My sis (Grobben [10]). In connection with the autennaland maxillary glands of Malacostraca generally it appearsthat the kidney part of the gland is able to undergo compli-cation in two different ways : (1) It may either elongate andbecome greatly convoluted (Asellus, Gammarus, Mysis), or(2) it may swell and its wall become invaginated by inwardextensions of the external hasmocoele. As examples of thelatter method may be mentioned the maxillary gland ofSquilla and the antennal glands of Cancer (Pearson [18]),Homarus (Waite[23]), Palsemonetes (Allen [1]), etc. I shouldlike here to record my suspicion that many descriptions (e.g.Waite's description of the antennal gland of Homarus ([23])and Weldou's description of the kidney region of certain otherDecapods [24]) of the structure of the kidney region of theantennal gland in different genera of Malacostraca commit theerror of describing as an elongated convoluted duct what should


be described as the extensive imaginat ion of a swollen sac. Inboth cases sections would reveal tubular structures cut across,aad it is only by carefal reconstruction and examination ofthe investing epithelia that the observer can be certain as towhich type of structure his sections represent.

Concerning the comparative histology of the maxillarygland of Squilla, I must remark tha t in general it resemblesthat of the antennal and maxillary gland of other Crustacea,as reference to the works of Grobben, Rogenhofen, Allen,Vejdovsky and others will show.

As regards the germ-layer origin of the antennal andmaxillary glands in Crustacea, it is interesting to note thedifferences of opinion among authors on the subject. Forinstance, my figure of the youngest stage of development ofthe maxillary gland of Squilla closely resembles those ofIshikawa ([11], his fig. 92) and Grobben (10), representingthe early development of the antennal glands of Atyephiraand Cetochilus respectively, but whereas Ishikawa states thatthe young gland is ectodermal in origin, Grobben holds thatit is mesodermal, arising as an invagination of the somato-pleure. Reichenbach (19) held that the antennal glands ofAstacus are purely ectodermal structures, and a number oflater authors agree that at least the external par t of thegland in the Crustacea is of ectodermal origin, e . g . Ishikawa(11) believes that the entire gland of Atyephira is ectodermal,whilst Lebedinsky (15;, Boutchinsky (2) and Waite (23) statethat the duct and kidney are ectodermal, the end-sac beingmesodermal. On the other hand, Grobben (10), Kingsley(13) and Robinson (20) consider the whole gland to be amesodermal product, but Grobben's statement concerning thespmatopleuric invagination is certainly difficult to believe,since a ccelomoduct is formed as an outpushing, not aningrowth, and Robinson's figures of the first rudiments of theantennal gland in Nebalia show that the initial clump of cellsmay just as well he considered ectodermal as mesodermal.I t must be concluded, I think, that the evidence for theectodermal origin of the duct ;md kidney of the gland is pre-


ponderant. Concerning the origin of the end-sac, the evidenceis not so one-sided. The statements of Waite, confirmingthose of Lebedinsky and Boutchinsky, are quite definite as tothe origin of the end-sac in a distinct mass of mesoderm cells,and the secondary junction of the end-sac with the ecto-dermal kidney and duct. On the other hand, Grobben figuresthe end-sac cells as forming the upper extremity of the simpleinvagination in the early development of the gland, just as Ihave figured these same cells in my drawing of the youngeststage of the Squilla gland—there is no separate origin of theend-sac here—and Boutchinsky merely asserts that the deepend of the ectodermal invagination becomes surrounded bymesoderm cells—a statement which may have resulted froma mistaken conception of the origin of the "mesoderm" cells.Further, Waite's supplementary argument for the separateorigin of the end-sac based upon the entirely distinct cha-racter of the cells of the end-sac as compared with those ofthe kidney counts for little; my description of the differencesof the kidney and duct-cells and their abrupt distinctness (notransitional kinds of cell existing) in the gland of Squilkmight equally well be employed to suggest that the kidneyand duct regions ai*e of separate origin. In view, then, of thescanty and conflicting evidence on the subject, we are not ina position to draw conclusions respecting the ectodermal ormesodermal origin of the end-sac, still less to discuss thehomologies either of the various parts of the antennal andmaxillary glands or of the glands as a whole. Future inquirycan alone decide whether or not these Crustacean leg-glands(rudiments of which, it must be remembered, have been foundiu connection with the maxillipedes [15] and other thoraciclimbs in addition to the second antenna and second maxilla)are to be compared with the ectodermal crural glands ratherthan with the mesodermal excretory organs of Peripatus asheretofore.

INTERNAL ANATOMY OV SQUILLA. 423

II . NOTES ON SOME OTHEE FEATURES OF THE INTERNAL ANATOMY

OP SQUILLA.

The Communications between the Pyloric Diges-tive Gland ("Liver") and the Gut.

Johannes Muller (29) in 1830 made the statement that theso-called "l iver" possessed numerous pairs of segmentallyarranged openings into the alimentary tract, and later authors(e.g. Duvernoy, 1836 [26], Milne Edwards, 1859 [27], Ger-staecker, in Bronn's ' Thierreich,' 1889 [31] and others) haveeither accepted thi3 statement on Mulleins authority or havesupposed that their reinvestigations have confirmed Muller.However, in 1901 Orlandi (30) to a large extent correctedthis erroneous idea, and stated that the two halves of tlie" liver " solely opened into the gat by a single median openinginto the dorsal side of the pyloric part of the stomach, and thisstatement has appeared in most recent English text-books,e. g. in Caiman's " Crustacea" in Laukester's ' A Treatise onZoology ' and Sedgwick's ' Student's Text-book of Zoology.'It must be pointed out, however, that Fritz Muller (28) in1863 clearly described and figured a single pair of " l iver"ducts opening into the sides of the stomach in an Erichthuslarva, and this fact would by itself lead us to suppose that theadult condition is similar to that of the larva.

A further correction concerning this much misinterpretedand, indeed, trivial point of fact has now to be made in thepresent paper. Orlandi was correct in stating that each half ofthe "liver" only possesses a single duct opening into the pyloricregion, but quite wrong in stating that the two ducts unitetogether and open by a single mid-dorsal aperture. Examina-tion of complete series of transverse and longitudinal sectionsthrough the adult Squil la desmares t i i , Risso, shows thateach duct opens independen t ly into the side of the pylorusby a large triangle-shaped aperture (the apex of the isocelestriangle being anterior and the vertical base posterior), andso conforms to the arrangement found in most Crustacea. In

424 W. K". F . WOODLAND.

front of these lateral openings of the pyloric gland into thestomach, the gland is produced into two blind diverticulawhich extend forwards as far as the base of the second

TEXT-FIG. 9.

Dorsa.1.

-,-S.L.C

LD

- , — G . C

--S.L.C.

- , - L C

/ Gkst.CCh. A. Ventral

Transverse section across pylorio region of stomach, showingthe two lateral openings of the pyloric digestive gland (explainedin the text). L. 0. Anterior extremity of left opening of glandinto stomach. Qast. C. Gastric cells. L. Lum. Lumen ofdigestive gland. L. D. Small dorsal diverticnlum of gland.P. S. Cavity of pyloric stomach. Ch. A. Chitmous lining ofstomach. S. L. C. Small cells. G. C. "Goblet" cells. ( x 173.)For other reference letters see p. 429.

thoracic limb. Each of these anterior diverticula is joineddorsally by a smaller diverticulnm (L. D. in Text-fig. 9) at thelevel of the posterior end of the gland opening into the stomach.

INTERNAL ANATOMY OP SQU1IYLA. 425

Posteriorly to this opening the gland extends back on eachside as a wide sac in close contact with the narrow intestine(which is thus effectively hidden from view), and gives offnumerous lateral diverticula in its course. These diverticula;ire somewhat narrow proxiinally but dilate and branch dis-tal ly; in the sixth abdominal segment they extend into thel>asal joints of the appendages borne on that somite, and theyoccupy the greater part of the cavity of the telson. Text-fig.9 represents on its left side a section passing through theanterior extremity of the opening of the pyloric gland intothe stomach, which is thus seen to be narrow and ventro-lateral; the right side of the same figure passes through theposterior extremity of the opening, which is thus evidentlywide and lateral. Sections also show that the whole of thegland is enveloped in a large blood-sinus containing muchblood.

The cells of the digestive glands are of several kinds, themost noticeable being the ordinary granular cells (L. C), cellswedged in between the ordinary cells and containing largevacuoles (G. C.; these vacuoles burst into the gland cavity) andsmall cells with very large darkly-staining nuclei (8.L. C).

The Rectal Glands.

The rectal glands are found in both sexes underlying theramifications of the "liver" in the telson. They are two innumber, are large, with spacious lumina, and open in the adultus in the larva (see Olaus [5]), laterally at the posterior endof the rectum. In addition to these rectal glands there arepresent some small accessory tubules opening into the gut inthe same region, which apparently bear some resemblance instructure to the urinary tubes of Amphipods.

The mesenteron is very long in Squilla and extends fromthe stomach to the region of the auus, where the cuticle ofthe proctodteum becomes inturned for only a short distance.


The Naupl ius Eye.

The Nauplius eye persists in the adult Squilla, and caneasily be seen under a low-power binocular as a small patchof black pigment lying towards the ventral side of the headbetween the bases of the paired stalked eyes. The pigmentedeye apparently contains in the specimens examined eithertwo or three ocelli (probably three, since this is the numberfound in most Crustacea), and is connected by a single slendernei-ve to the brain.

APPENDIX.

Methods of P repa ra t i on of Mater ia l .

The adult specimens of Squil la desmarest i i were fixedfor me at Naples in Hermann's fluid, in Zenker's fluid, incorrosive-acetic and in hot absolute alcohol. On receivingthem I carefully decalcified certain specimens in a mixture ofnitric acid (over 5 per cent.) in alcohol (the liquid constantlyrenewed) for three or four weeks. They were then em-bedded in hard (60° C.) wax and cut into sections 10 (i thick.Despite the thickness of the cuticle, complete series of trans-verse and longitudinal vertical sections were obtained ofthree entire animals. Complete series of transverse andlongitudinal sections were also made of many larvse of thesizes mentioned in the above description. All these sectionswere double-stained on the slide in Ehrlich's h£ematoxyliu(twenty-four hours), followed by picro-indigo-carmine. (Addone part saturated solution of picric acid in 90° alcohol totwo parts saturated solution of Griibler's indigo-carmine in70 per cent, alcohol, aud dilute this stain with equal bulk of70 per cent, alcohol; for mode of using this stain see mypaper on " Fish Gas Glands," ' Proc. Zool. Soc. Lond.,' 1911.)(Sections of the Hermann's fluid material were found to bebest preserved.


LITERATURE RELATING TO THE MAXILLARY GLAND.

1. Allen, E. J.—" Nephridia and Body-cavity of some DecapodCrustacea," ' Quart. Journ. Micr. Sci.,' vol. 34,1892-93.

2. Boutchinsky.—' Mem. Soc. Nat. New-Russia,' T. xix (Part 2), 1895.

3. Caiman, W. T.—The Fascicle on " Crustacea" in Lankester's' Treatise on Zoology,' 1909.

4. Clans, C.—' Untersucliungen zur Erforschung der genealogischenGrundlage des Crustaceen-systems: ein Beitrag zur Descendenz-lehre," Wien, 1876.

5. " Die Kreifslaufsorgane und Blutbewegung der Stomato-poden," 'Arb. Zool. Inst. Univ. Wien,' Bd. v, 1883.

6. " Untersuchungen iiber die Organisation und Bntwickelungvon Branchipus und Artemia, nebst vergleichenden Bemerkungeniiber andere Phyllopoden," ' Arb. Zool. Inst. Univ. "Wien,' Bd. vi,

7. " Ueber den Organismus der Nebaliden und die systematisclieStellung der Leptostraken," ' Arb. Zool. Inst. Univ. Wien,' Bd. viii,1889.

8. — "Ueber Apseudes Latre i l l i i , Edw. und die Tanaiden,"' Arb. Zool. Inst. Univ. Wien,' Bd. ix, 1891.

9. Defner, A.—"Der Bau der Maxillardriise bei Cirripedien," 'Arb.Zool. Inst. Univ. Wien,' Bd. xviii, 1910.

10. Grobben, C.—"Die Antennendriise der Crnstaceen," 'Arb. Zool.Inst. Univ. Wien,' Bd. iii, 1881.

11. Isliikawa, C.—" On the Development of a Freshwater MacrurousCrustacean, Atyephira compressa, DeHaan," 'Quart. Journ.Micr. Sci.,' vol. 25, 1885.

12. Jurich, Bruno.—"Die Stomatopoden der deutschen Tiefsee-Expedi-tion," ' Wissenschaft. Ergeb. Deutsch. Tiefsee-Expedition auf demDampfer "Valdivia," 1898-99,' Bd. vii, 1904.

13. Kingsley, J. S.—"The Development of Crangon vulgaris,"(third paper), 'Bull. Essex Inst.,' vol. xxi, 1889.

14. Kowalevsky, A.—" Ein Beitrag zur Kenntnis der Exkretionsorgane,"'Biol. Centralblatt,' Bd. ix, 1890.

15. Lebedinsky, J.—'Mem. Soc. Nat. New-Russia,' t. xiv (Part 2),1889.

16. Lister, J. J.—" Note on a (? Stomatopod) Metanauplius Larva,"'Quart. Journ. Micr. Sci.,' vol. 41,1898.

428 \Y. N. V. WOODLAND.

17. Marchal, P.—" Recherches Anatomiques et Physiologiques surl'Appareil Excreteur des Crustaces Decapodes," ' Arch. d. Zool.Exp. et Generate,' Paris, Ser. 2, T. x, 1892.

18. Pearson, J.—Memoir on Cancer in the Series of the LiverpoolMarine Biological Committee, 1908.

19. Reichenbach, H.—" Die Enibryonalanlage und erste Entwicklungdes Plusskrebses," ' Zeit. f. wiss. Zool.,' Bd. xxix, 1877.

20. Robinson, M.—" On the Development of Nebalia," ' Quart. Journ.Micr. Sci.,' vol. 50, 1906.

21. Rogenhofer, A.—" Zur Kenntnis des Baues der Kieferdriise beiIsopoden und des Grossenverhaltnisses der Antennen- undKieferdriise bei Meeres- und Siisswasserkrustazeen," ' Arb. Zool.Inst. Univ. Wien,' Bd. xvii, 1908.

22. Vejdovsky, F.—" Zur Morphologie der Antennen-und Schalendriisebei Crustaceen," ' Zeit. f. wiss. Zool.,' Bd. lxix, 1901.

23. Waite, P. C.—" The Structure and Development of the AntennalGlands in Hoinarus americanus, Milne Edwards," 'Bull.Mus. Comp. Zool. Harv.,' vol. xxxv, 1899-1900.

24. Weldon, W. P. R.—" The Renal Organs of certain DecapodCrustacea," ' Quart. Journ. Micr. Sci.,' vol. 32, 1891.

25. Woodland, W. N. P.—" On the Renal Organs and some otherFeatures of the Internal Anatomy of Squilla," ' Report (Section D)of Brit. Assoc. Advance. Sci.,' Portsmouth, 1911.

LlTEKATDKE DELATING TO THE P Y L O E I C DIGESTIVE GrLANDS

( " L I V E R " ) OF SQUILLA.

26. Duvemoy, G. L.—"Sur le Poie des Animaux sans Vertebres engeneral et particulierement sur celui de plusieura Crastaces,"' Ann. Sci. Nat,.' ser. 2, t. vi, p. 243, pi. 15, Paris, 1836.

27. Milne Edwards. H.—' Leijons sur la Physiologie et l'Anatomiecomparee," t. v, Paris, 1859, pp. 557, 566.

28. Miiller, F.—" Ein zweites Bruchstiick aus der Entwickelungs-geschichte der Maulfiisser," 'Arch. f. Nat.,' Bd. xxix, pp. 1-6,pi. 1, 1863.

29. Miiller, J.—'De Glandnlarum secernentium Stractura.' Lipsise,1830, p. 69.

30. Orlandi, S.—"Sulla Struttura dell' Intestiuo della Squillamant is (Rond.)," 'Atti. Soc. Ligustica,' vol. xii, pp. 112-133,2 plates, 1901.

31. Gerstaecker, A.—" Crustacea " in Bronn's ' Klassen und Ordnungendes Thierreichs,' Bd. v, Abth. 2, 2te Hiilfte, pp. 705-707, 18S9.

INTERNAL ANATOMY OF SQTJ1IXA. 429

EEFEBENCE LETTERS OF FIGURES (PLATE AND TEXT).

A. 1. Antermula. A. 2. Antenna. Bl. Bladder. B. S. Blood-space(hasmocoele). C. Carapace. C. B. Duct-cells. Ce. A. Cephalic aorta.Ck. A. Chitinous armature of stomach. Conn. Connective-tissue strands.C. P. Col. Connections between columnar cells and "plugs" in cuticle.C. Str. Striations in cytoplasm of kidney-cells. Cut. Cuticle. D. Mx. Gl.Duct of maxillary gland. Ds. Dorsal surface of gland. D. Sq. Ep.Double layer of squamous epithelium. E. Eye. End. S. End-sac. End.S. C. End-sac cells. Ep. Epipodite. Ep. 2. Epipodite of second thoracicappendage. Ex. Scale-like exopodite of antenna. Exc. Excretionmaterial. F. C. Free carapace. Gast. C. Gastric cells. G. C. Goblet-cells. G. D. Lateral diverticuhim of duct. Gl. Glands (" dvusenzellen").Gr. Groove on side of gland lodging body muscle. Grn.Granules. H.M.Hffimocoele. Ht. Heart. Hyp. Hypodermis. I. C. B. Invagination ofduct-cells. I. M. Invagination of kidney wall. / . P. Invagination ofpartition wall separating end-sac and kidney. I. Sq. Ep. Inner layer ofsquamous epithelium. / . Junction of gastric and hepatic cells. L. Pyloricdigestive gland ("liver"). L. C. "Liver" cells. L.D. "Liver " diverti-cuhim. L.Itimn. "Liver" lumen. L. 0. "Liver" opening into stomach.Lum. Lumen—excretory. L. W. L. 8. Apposition of "'liver'' and pyloriestomach on left side. M. C. Kidney-cells. M. Mx. Gl. Kidney portionof gland. Mx. Maxilla. Mx. Gl. Maxillary gland. Mxl. Maxillula.Mas. Muscles. Mus. Lon. Longitudinal muscles. Mus. Tr. Transversemuscles. Nc. Nerve cord. Op. Opening of maxillary gland on papilla.0. Sq. P. Outer layer of squamous epithelium. P. C. Protoplasmicprocesses from end-sac cells traversing blood-space. P. Col. Mass ofcolumnar epithelium at opening of maxillary gland-duct. P. S. Pyloricportion of stomach. B. Rostrum. S. L. C. Small special "liver"cells (resembling oxyntic cells). Str. B. Striated border of kidney-cells.8p. Ep. Squamous epithelium. Th. 1, Th. 2, Th. 3 . . . Th. 7. Thoracicappendages. TJ. Th. G. United ganglia supplying the mandibles andthe first five thoracic segments and bearing the circum-cesophagealconnectives anteriorly. V. Vacuole. W. El. Wall of bladder.


EXPLANATION OP PLATE 28,

Illustrating Mr. W. N. F. Woodland's paper, " On theMaxillary Glands and some other Features in theInternal Anatomy of Squilla."

[All figures of Squilla desmarest i i , and all drawn by means ofthe camera lucida.]

Fig. 1.—Anterior end of Squilla, dissected to show the maxillaryglands (Mx. Gl.) in situ. The carapace has been cut away dorsallyover a large area, and the underlying hypodermis. some large obliqueand longitudinal body muscles, the anterior caeca of the liver, and partof the gut and cephalic aorta have been also removed. (x 3.)

Kg. 2.—The maxillary gland (Mx. 01.) removed from the bodywith the second maxilla and viewed laterally. Its external aperture(Op.) on the papilla (situated on the outer or convex side of the limb),the bladder (SI.) and the mass of the gland can be seen. Most of themuscles entering the limb have been removed, and two are representedin outline by dotted lines. GL, small gland-cells. (x 6.)

Tig. 3.—Transverse section across the left side of Squilla in theregion of the maxillary gland. As mentioned in the text, the details ofthe gland structure are shown on an immensely magnified scale in orderto make apparent the general construction of the gland; in reality thecell-layers of the gland are much smaller and much more numerous.M. Mx. Gl. General (kidney) mass of gland. D. Mx. Gl. Dact of maxil-lary gland. Bl. Bladder portion of gland. Op. Opening of maxillarygland on papilla. P. Col. "Plug" of columnar epithelium at openingof duct (see Text-fig. S). (x 20.)

Mx. 61

,59,N.S.01.28.MUS.TT.

London..

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