ON THE STEUCTURE OF HYDBACTINIA ECHINATA. 77

On the Structure of Hydractinia echinata.

By

Margaret C. Collcutt,Zoological Laboratory, University College, London.

With Plate 1.

AN investigation of the structure and relative positions ofthe cosnosarc and chitinous parts of Hydrac t in i a echinatawas suggested to me by Professor Weldon, for whose generousadvice and assistance I cannot be too grateful.

From my observations on this hydroid I find that the chitin-ous skeleton is for the most part a continuous irregular crustattached to some foreign object, and overlaid by a coenosarcconsisting of two layers of ectoderm, enclosing between thema number of branching and anastomosing endodermal tubes,which are connected at intervals with the endodermal canals ofthe polyps, the upper layer of ectoderm being continuous withthe ectoderm of the polyps.

This view is somewhat similar to that of Strethill-Wright,who describes the chitinous skeleton as forming a continuouscrust below the coenosarc, which, according to him, consistsof two ectodermal layers enclosing a single and continuouslayer of endoderm permeated by tubular cavities.

With otber observers of Hydrac t i n i a the prevalent ideaas to the structure of the colony seems to have been that theskeleton was tubular, the chitinous tubes enclosing the coeno-sarc, which also extended as a continuous sheet over the surface

78 MARGARET 0. COLLCUTT.

of the skeleton, and here connected the polyps with oneanother.

The earliest reference we have t o H y d r a c t i n i a i s that com-prised in a few lines by F leming (1) in 1828; this observer,however, regarded the horny crust of Hydract in ia as beinga polyzoon ectocyst, and assigned to it the name Alcyoniumechina tum.

This view of the nature of the hydroid was also held byJohnson (2) in 1838.

Van Beneden (4), in 1841, first gave the generic name ofH y d r a c t i n i a to the hydroid, and wrote a paper on thestructure of the egg. He regarded the sporosacs borne roundthe sides of the reproductive polyp as " eggs," but gave nodescription of the structure of the colony.

Hassal l (5), in 1841, described, under the name " Echino-chor ium clavigerum," a hydroid found in Dublin Bayadherent to empty univalve shells, and which is, according toAllman (13), the " H y d r a c t i n i a , sp." of Van Beneden.According to Hassall the colony consists of a " polypidommuricated with rough, spinous papillae about a line in height.There are numerous indentations on the surface of the polypi-dom, in each of which the base of a polyp is inserted; thislatter is about a quarter of an inch in height, and is of a whitecolour; its head is somewhat enlarged, and is surrounded withnumerous contractile club-shaped tentacula; the number ofthese varies considerably, but frequently amounts to betweentwenty and thirty. The tentacles are not arranged in anydeterminate order, but are variously disposed. Whether thepolyps are separate or united at their bases I am unable tosay." His accompanying figure represents nutritive polypsand ridged spines on a reticulated base.

Ph i l ipp i (6), in 1842, described as " Dysmorphosa con-chicola " a hydroid from the Bay of Naples; his descriptionis short, but he recognised the fact that the polyps are con-nected at their bases.

De Quatrefages (7), in 1843, described Hydrac t in ia underthe name of Synhydra paras i tes in the most comprehensive

ON THE STRUCTURE OF HYDRACTINIA EOHINATA. 79

paper on this hydroid that had so far appeared. He mentionstwo polyps,—reproductive polyps without a mouth, and nutri-tive polyps with a mouth; but describes the chitin as an endo-skeleton developing in the substance of the common coenosarc.He says that in the nutritive polyps the tentacles vary innumber, are distributed in two alternating circles, and arevery contractile. The polyps are all connected with a commonliving tissue which is directly continuous with the exteriorlayers of the body, and which, towards the edge, only consistsof a delicate pellicle enclosing no solid skeleton. Below thehorny base, and protected by it, there are little anastomosingtubes, the central canals of which communicate with thedigestive cavity of the polyps.

De Quatrefages is the first observer who describes thereproductive polyps in any detail.

Van Beneden (8), in 1844, published a second paper, inwhich he describes the two sexes of Hydrac t in i a as twodistinct species—H. rosea and H. lactea.

Johns ton (9), in 1847, revived the specific name " e c h i -na ta , " used by Fleming. He did not accept De Quatrefages'view of the endoskeletal nature of the chitia; on the otherhand, he conveys no distinct impression as to how the indi-viduals are connected together.

S t r e t h i l l - W r i g h t (10), in 1856, published a paper onHydrac t in ia , containing the most correct account of thestructure of the colony which had so far appeared. Hedescribes the corallum (skeleton) as being secreted externally,and forming a raised network between the spines. The poly-pary (soft tissue) " invests the corallum and fills up the grooves" of its papillae, the interstices between its reticulations, and" the cavities of its hollow spines. It is often absent at the" summits of the papillae. It secretes, renews, and extends the" corallum, gives rise to new polyps, and is the seat of commu-" nication between the polyps of the colony. . . . I conclude" that the polypary of this zoophyte consists of a single layer"of endoderm enclosed between two layers of ectoderm. That" the lower ectodermic layer, as it grows over the shell, attaches

80 MARGARET C. OOLLCUTT.

"itself by its colletoderm" (which is, according to Strethill-Wright, an epidermis replacing the corallum at the growingextremities of the branches), " and secretes the horny plate of" the corallum. On this plate, by a further process of secretion"from the lower ectoderm, the grooved spines are erected." That the upper layer of ectoderm is naked over the greater" part of its surface, or only covered by a thin epidermis; but" occasionally this layer also takes its share in the secretion of" the corallum, and in that event produces the smooth conical"spines, the concavity of which it fills."

Strethill-Wright conveys the impression that the endodermof the polypary is a continuous layerj permeated by. tubularexcavations, which are connected with the digestive cavitiesof the polyps.

With respect to the edge of the colony he says, " On theless exposed parts of the shell the polypary frequently passesbeyond the papillary corallum as a thin membranous expan-sion, or breaks up into a loose network of delicate anasto-mosing tubes Propagative stolons are given off bythese tubes ; a delicate chitinous investment mayalso be detected on the creeping tubular fibres, from which thestolons of Hydrac t in ia take their rise; but I have notsatisfied myself as to its presence on the entire upper surfaceof adult polyparies."

Two kinds of neniatocyst, differing in size, are mentionedby Strethill-Wright, who is the first observer of the spiral andtentacular polyps. He describes the spiral polyps as mouth-less, with rudimentary tentacles and a highly developedmuscular coat, large nematocysts being crowded in the ecto-derm of the tentacles and the whole body. The tentacularpolyps he regards as alwa}'S present. " Their tips are coveredwith a dense pavement of the larger thread-cells, and a fewof the same bodies are thinly scattered along their wholelength."

Van Beneden (11), in 1866, published another paper onHydrac t in i a , in which he says that the tentacles are gener-ally absent in the reproductive polyps. What he formerly

ON 0)HE STBUOTURS OF HYDltACTItflA E0H1NATA. 81

described as "eggs" he now designates "sporosacs," andasserts that the skeleton is external and similar to that ofCampanula r ia and Tubular ia . The reproductive polypshave no mouth and only rudimentary tentacles. He confusesHydrac t i n i a with Podocoryne, and distinguishes threenew species of Hydrac t in ia .

Hincks (12), in 1868, also mentions tentacular polyps ; he,as well as Strethill-Wright, describes fixed reproductive sacs.He speaks of the basal cosnosarc as consisting of " a numberof anastomosing tubular stolons closely packed together, andfilling in the tubular orifices of the chitinous skeleton, whichlatter appears to consist of a series of tubes laid side by sideon a plate of chitin, and closely appressed one to the other."

Al lman (13), in 1871, published a full account of Hy-dract inia , in which he affirms that the basal part of thecolony consists of a number of closely approximated chitinoustubes containing coenosarc, which consists mainly of endodermwith only a thin investment of ectoderm, the latter secretingthe chitin. " At the free surface of the coenosarcal expansiouits intercommunicating canals are only partially invested bychitin, this excretion being in the superficial layer of canalsconfined to the deeper parts, thus forming open channels inwhich the canals are lodged, so that when the soft parts areremoved the chitinous perisarc forms on the surface a multi-tude of intersecting ridges, having between them the channelswhich had contained the superficial cosnosarcal canals. Uponthe whole of the free surface, however, the ectoderm of thesecanals forms a continuous and very conspicuous layer, havingacquired increased thickness and developed in its substanceabundance of thread-cells. The whole free surface of thecommon basal expansion of the colony thus presents an abso-lutely naked layer, of ectoderm. . . . There can be no doubtthat the whole hydrophytoii, of Hydrac t in ia must be re-garded as consisting of a set of ccenosarcal, freely intercom-municating tubes, which have excreted from their free surfacea chitinous perisarc, and have intimately coalesced with oneanother."

VOL. 4 0 , PART 1.—NEW BEE. F

82 MAKGAREL' 0. COLLCUTT.

An injured part of the hydrophyton repairs the injury bya network of coenosarcal tubes invested by a chitinous perisarc,the meshes of which are ultimately obliterated by the thicken-ing and coalescence of their chitinous walls. The ectodermand endoderm of the hydranths is continuous with the ecto-derm and endoderm of the basal coenosarc.

The spiral zooids are long, cylindrical, mouthless hydroids,with a crown of rudimentary tentacles crowded with largenematocysts; they have a tubular eudodermal cavityj andpossess the power of coiling themselves into a spiral: themesogloea fibres are strongly developed. The blastostyles mayhave a mouth.

Allman believes that the tentacular polyps are only rarelypresent, and regards them as abnormal modifications of otherhydranths.

Wei smann (15), in 1883, described the blastostyles andmigration of the sex-cells in Hydractinia. He figures a blasto-style with a mouth and ciliated endoderm, and mentions theoccurrence of food granules in the endoderm of the upperregion of the body.

Miss B u n t i n g (17), in 1894, published an account of theorigin of the sex-cells and the development of Hydractinia.She first observes the ova in the endoderm of the blastostyle,and concludes that they are probably endodermal in origin.

My observations on the structure of H y d r a c t i n i a echi -na t a have led me to agree to a certain extent with Strethill-Wright with respect to the relations of the ccenosarc andchitin, and to differ from Allman and Hincks as to their viewsof the tubular nature of the adult skeleton.

Moreover I have found a dactylozooid with a mouth, theexistence of which has hitherto been overlooked or denied,while other observations show that there is a migration ofova between ectoderm and endoderm in the blastostyle.

Colonies of Hydractinia are generally situated on the surfaceof shells,•which are commonly whelk-shells inhabited by hermit-crabs. A large colony may cover the whole shell except fora small roundish patch where the shell rubs along the ground

ON THE STRUCTURE OF HYDRAOTItflA EOHINATA. 83

as the hermit-crab drags its home about; a small colony isusually situated near the edge of the shell.

A Hydractinia colony is comprised of four kinds of polyps :1. Gasterozooids, or Nutritive polyps.2. Blastostyles, or Reproductive polyps.3. Dactylozooids, or Spiral polyps.4. Tentacular polyps.

The Gasterozooids are the most numerous of the polyps ; inthe early spring and in the summer, however, the Blastostylesincrease greatly in numbers, and at these times give rise tothe generative products, the colonies being either male orfemale.

Bound the shell mouth are situated the Dactylozooids,which are capable of coiling themselves spirally, and mayfunction as defensive polyps.

Strethill-Wright (10) and Hincks (12) also mention "Tenta-cular polyps," which are scattered on the outskirts of thecolony, and which they regard as constant in occurrence.Allman (13), on the other hand, does not believe that theyare universally present, and supposes that they are merelyabnormalities when they do occur.

However, in all those colonies which I have closely exa-mined with a view to finding these polyps I have beensuccessful; in one specimen they were particularly abundant,occurring principally at the growing edge of the colony. Inother specimens they were rare, while on one very flourishingcolony I only found about half a dozen. I doubt if they areabnormal dactylozooids, as Allman suggests, for I have neverfound them situated on that part of the colony frequentedby dactylozooids.

These various kinds of polyp are all connected together attheir bases by a common cceuosarcal expansion, which is awhite semi-transparent investment completely covering thechitinous skeleton.

I succeeded in keeping several colonies alive for some timeby suspending the shells on which they were situated in smalltanks, the other occupants of the sheila being removed. One

84 MARGABET C. COLLCUTT.

of these colonies lived in this way for about three mouths,when the zooids gradually dwindled away, and finally all disap-peared. This colony, as is general with Hydractinia, ex-tended itself round the shell mouth into the interior of theshell, and apparently began to absorb this enclosed portion ofthe shell, which became thin and brittle.

In other colonies kept in confinement the dactylozooidswhich were congregated round the edge of the shell disappearedafter a few days.

A point of some interest which I noted in examining whelk-shells tenanted by hermit-crabs and bearing Hydractiniacolonies on their surfaces is that a Polychsete worm, Nere isbil ineata, was always found living inside the shell in com-pany with the hermit-crab. This worm lived in the coils ofthe tip of the shell, and could completely withdraw itself fromobservation.

The most successful killing reagents used were Flemming'ssolution and picric acid solution; Hermann's solution alsogave good results for histological purposes. The staining re-agents principally used for sections were Delafeld's hsema-toxylin and borax carmine. For surface views of the thin edgeof the colony aniliue orange was found to be an excellent chitinstain; the preparations were left for a few minutes in a 90 percent, alcoholic solution of aniline orange, when the chitin wasstained bright yellow, the protoplasm being but slightlystained.

GENERAL ANATOMY.

The Skeleton.—This is a continuous unevenly depositedlayer of horny chitin, so closely attached to the rough surfaceof the shell that the latter must be decalcified in order toisolate the hydroid colony. It is, for the most part, secretedby the lower layer of ectoderm which forms part of the cceno—sarc of the colony. In the central parts of the colony it isin many places of considerable thickness, with irregularlacunae, and thickly beset with small chitinous spinules, whileit is frequently raised up into a number of large conical

ON THE STRUCTURE OP HYDEAOTINIA EOHINATA. 85

grooved spines. Even, in these mature parts of the colony,that part of the skeleton which connects the spinules andspines together is often exceedingly thin and inconspicuous.

The spinules are merely solid chitinous thickenings, irre-gular in shape and size, projecting upwards from the chitin•which is attached to the whelk-shell. They occur all over theskeleton, and are very conspicuous towards the thin edge ofthe colony (PI. 1, fig. 1, a and b).

The spines are one of the characteristic features of thecolony; they are mostly deeply furrowed, having longitudinalserrated ridges between the grooves. The ridges are con-nected with one another by chitinous cross-bars, and fusetogether at the tip of the spine. Central chambers or a singlechamber communicate with the grooves. The whole spine iscovered by the general coenosarc, which is also continued intothe meshes of the framework, though it may often becomerubbed off from the tips of old spines where the shell hascome in contact with the ground.

Allman (13) has figured and described a typical spine inhis account of H. ech inata. Sometimes the spines bifurcatetowards their apex, or are otherwise irregularly shaped.

Between the spines and spinules the skeleton is in someplaces deposited as a single thin layer, but it is usually secretedin thin, irregular, reticulated layers, one above another. Inthe older parts of the colony these layers, in vertical section,have the appearance of chitinous strands varying in thicknessand running horizontally or obliquely (PI. l,fig. 1, c), but towardsthe thin growing edge of the colony they are more regular.

At intervals throughout the skeleton there are spaces sur-rounded on all sides by chitin, and containing degeneratingmasses of coenosarc; such masses have become constricted offfrom the rest of the coenosarc by unequal growth of the chitin,the various stages of such constriction being demonstrable.In some cases where the constriction and isolation are not com-plete the degenerating coenosarc is seen to be in direct con-tinuity with the coenosarc of the colony (PI. 1, fig. 2). In otherpases the spaces are empty.

86 MARGARET 0. COLL0UTT.

The chitin which is situated beneath such a degeneratingmass becomes somewhat cup-shaped by unequal growth (PI. 1,fig. 2). Gradually the edges of the cup grow over, only leavingthe mass in the cup attached by a narrow bridge to the uppercoenosarc, until finally the edges of the cup meet and coalesce,and so isolate the degenerate mass of coenosarc. These con-stricted masses consist only of the deeper parts of the cceno-sarc, i. e. the lower ectodermal layer and the endoderm, and asthey degenerate the boundaries between the cells becomeobliterated, the protoplasm becoming very granular, and thenuclei losing their distinctive appearance.

The Edge of the Colony.—At the edge of the colonywhere growth occurs the skeleton is very thin, though thick-ened at frequent intervals into short, solid spinules; here, too,the coenosarc is generally thinner, and the polyps are few andsmall.

Extending over the surface of the coenosarc of this region isa thin membranous layer of chitin, which is connected atmany points with the tips of the spinules of the skeleton.

In this region the skeleton is occasionally raised up intoconical smooth spines, which are fenestrated at their bases, sothat portions of the basal ccenosarc, consisting of lower ecto-derm and of endoderm, can penetrate into the interior of thespines (PI. 1, fig. 3).

These spines are partially covered by ccenosarc, which, how-ever, does not extend over their tips; a thin layer of chitinspreads over the surface of the ccenosarc, but is not continuedover the naked tips of the spines (PL 1, fig. 3).

Strethill-Wright (10) mentions hollow chitinous spines filledwith coenosarc, but he describes them as being derived onlyfrom the upper layer of ectoderm.

The colony is derived at its growing edge from a number ofcoenosarcal tubes clothed with a chitinous perisarc; thesetubes branch about irregularly over a large portion of thewhelk-shell, into the cavity of which they extend ; ectodermcovers their tips, and they appear to secrete their perisarc asthey grow.

ON THE STRUCTURE OF HYDEACT1NIA. EOEINATA. 8V

The lower surface of the perisarcal tubes is thicker than theupper surface; this and the side walls are frequently thickenedinto vertical, conical spinules, and form the foundation of thecharacteristic skeleton of older parts of the colony. The thinupper surface has no chitinous thickenings, and usually stainsmore readily than the rest of the perisarc. As the cceaosarcaltubes continue to grow they branch repeatedly and anastomosewith one another until the tubular character of the chitinousinvestment is obliterated; in this way a sheet of coenosarc,enclosed between two layers of chitin, is formed.

When two such tubes anastomose, it is probable that partof the internal lining of ectoderm in each tube which liesnearest to the region of contact absorbs the chitinous wallalong this region ; consequently the two tubes are here placedin communication with each other, and the ectoderm of theupper half of one tube becomes continuous with the ectodermof the upper half of the other tube, while the ectoderm of thelower half of the one becomes continuous with the ectodermof the lower half of the other. Hence arises the differentia-tion of two ectodermal layers so characteristic of the matuTerregions of the colony (PI. 1, figs. 1 and 4).

The endoderm retains its tubular character.The chitinous spinules along the side walls of anastomosing

tubes do not all become absorbed, but many remain as part ofthe permanent skeleton, and are attached at their tips to thethin upper membrane of chitin (PI. 1, fig. 1, a, c, b).

As the colony grows this membrane gradually weakens, andfinally is lost (PI. 1, fig. 1, b); the coenosarc then grows upover the tips of the spinules, and so becomes a continuousexpansion over the surface of the skeleton.

Here and there are empty chitinous tubes from which theprotoplasm seems to have withdrawn itself; frequently all thatremain of these tubes are the small chitinous spinules.

At intervals a small polyp branches from this growingregion, breaking through the upper chitin, which does notextend over the polyp above its base. In some of the coloniesI examined I found many " tentacular polyps" in this region.

00 MARGARET 0. OOLLCUTT.

The Basal Ccenosarc of the Colony.—As mentionedabove, this consists of two layers of ectoderm, enclosing be-tween them a ramifying mass of endodermal tubes, and formsthat part of the colony which connects the polyps with oneanother. The lower layer of ectoderm secretes the greaterpart of the skeleton, forming a continuous layer in contactwith it j the upper layer of ectoderm also forms an unbrokenexpansion, and is continuous with the ectoderm of the polyps.The two ectodermal layers thus form two sheets of cells, oneabove the other, and are in close contact with one another,except where the tubes of endoderm are interposed betweenthem, such tubes serving to place the cavities of the polyps incommunication (PI. 1, fig. 1, c).

These endodermal tubes are more or less oblong in cross-section, and are always surrounded by a narrow layer ofmesoglcea, which separates the endoderm from the ectoderm(PI. 1, fig. 5), and is continuous with the rnesoglcea of thepolyps.

At frequent intervals thread-like strands of mesogloea fromthe lower surface of this endoderm pierce the lower ectoderm,and attach themselves by slightly widened bases to theskeleton (PI. 1, figs. 1, c, and 5).

Similar strands fix the polyps to the chitin, but these arethicker, more numerous, and, since the lower ectodermal layeris generally much shallower below the polyps, they are oftenshorter. These processes thus exhibit the singularity of beingsurrounded on all sides by ectoderm cells. Similar strandsare figured by Weismann (15) as attaching the coenosarc ofEudendr ium ramosum to the perisarc.

The lower ectoderm and the endodermal tubes penetrateinto and fill up the intercommunicating chambers and thegrooves of the large spines, the superficial ectoderm extendingover all.

Strethill-Wright (10) and Allman (13) both mention thefact that this basal ccenosarc is capable of communicating theeffect of any shock it may receive to the various members ofthe colony.

ON THE STRUCTURE OF HYDRAOTINIA ECHINATA. 89

The Polyps.

1. The Gasterozooids.—The external appearance of thesepolyps has been frequently described and figured; they arecylindrical in shape and very contractile, and may attain to aheight of about a quarter of an inch. They are provided witha conical hypostome, terminating in a mouth, and havinground its base two closely approximated series of tentacles,which increase in number with the age of the polyp, and varyfrom ten to thirty in different colonies. These tentacles arevery contractile, and are provided with numerous nematocysts.The polyps are capable of everting their hypostomes. Theoutermost cell-layer of a Gasterozooid is a layer of ectoderm,which is continuous at the base of the polyp with the upperectodermal layer of the basal coenosarc; the alimentary canalis lined by a single endoderm layer, which forms a widecylindrical tube, connected at its base with the plexus of en-dodermal tubes which permeates the basal coenosarc (PI. 1,fig. 1, c). Between the ectoderm and endoderm is the well-marked mesoglcea.

2. The Blastostyles.—These are specially modified re-productive zooids, giving rise to " sporosacs." The blasto-styles are smaller than, and differ considerably in appearancefrom the Gasterozooids; they have a small mouth at the apexof a conical " head," and at a short distance from this terminalmouth there are two or more closely approximated circles oftentacles. These tentacles vary in number from ten to thirty;they are rudimentary kuob-like structures, containing prolon-gations of the endoderm and mesoglcea of the polyp, and theirectoderm is crowded with nematocysts (PI. 1, fig. 6).

Below the tentacles the body narrows gradually, becomingexternally constricted into a narrow " neck ; " then it widensconsiderably into a globular dilatation, from the walls of whicharise the round or oval sporosacs. The sporosacs are out-growths of the walls of the polyp, all three layers of the polypbeing involved in their formation : generative cells arise inthe body of the blastostyle and migrate into the sporosacs.

90 MAKGABET 0. COLLOUTT.

Below the globular region the body again narrows andbecomes connected with the basal ccenosarc. Miss Bunting(17) has fully described and figured the sporosacs and gene-rative cells of Hydractinia. She mentions that she was unableto trace egg-cells in the ectoderm or mesogloea of the blasto-style. I have, however, observed small cells situated in theectoderm of the blastostyle close to the mesogloea, and indis-tinguishable from undoubted egg-cells in the endoderm of thesame region of the blastostyle (PI. 1, fig. 6).

Also in two cases I have found egg-cells in the mesoglosa;the endoderm of the same region already contained severaleggs (PL 1, fig. 7). In one of these cases the blastostyle had notyet formed sporosacs.

3. The Dactylozooids.—These are cylindrical through-out their length, and are furnished at the distal extremity witha circle of knob-like rudimentary tentacles, from ten to sixteenin number, and crowded with nematocysts.

They have no hypostome, and are described by Allman (13)and Strethill-Wright (10) as having no mouth. I have, how-ever, found a distinct mouth situated in the centre of thetentacle circle, and leading down through a short tubnlar canalbounded by ectoderm into the endodermal cavity (PI. 1, fig. 8).These polyps are exceedingly muscular, and are capable ofcoiling and uncoiling themselves.

In one colony which I examined at Plymouth the dactylo-zooids were frequently branched once or twice, each branchterminating in a typical dactylozooid head.

4. The Ten t acu l a r Polyps.—These polyps are principallysituated towards the outskirts of the colony, though they mayalso occur in the older regions; most frequently I have foundthem arising from the network of tubes at the growing edge.They are exceedingly slender, though often longer than theother zooids comprising the colony, and are contractile, fortheir ectoderm is frequently marked by transverse folds. Theypossess a tubular internal cavity lined by large endoderm cells :I have not succeeded in demonstrating a mouth in theseindividuals.

ON THE STRUCTURE OF HYDRA0T1MA E0H1NATA. 91

The distal extremity is slightly enlarged into a club-shapedtip, covered all over with nematocysts, which also occur inconsiderable numbers throughout the ectoderm of the polyp.

la examining colonies of Hydractinia for these polyps Icame across two abnormal individuals. One of these had theappearance of a tentacular polyp which had given rise to twobranches situated at different levels, and each terminating, likethe main polyp, in a rounded tip covered with nematocysts.(Process, Fig. 1.)

The other abnormality was a gasterozooid from which, justbelow its tentacles, branched a tentacular polyp. (Process,rig. 3.;

FIG. 1. FIG. 2.

HISTOLOGY.

A. Histology of the Lower Ectoderm.

This layer extends, renews, and secretes the greater part ofthe skeleton ; occasionally, in very limited areas, it is morethan one cell deep.

The cells comprising it are not uniformly equal in size;beneath the polyps and towards the edge of the colony theyare more or less square and regular in shape, but in the adultparts they are frequently deeper than they are broad. As aconsequence of the great variation in the depth of the cells of

92 MAKGARET 0. 00LL0UTT.

this layer the thickness of the ccenosarc varies considerablythroughout the colony ; over the tips of the chitinous spinulesand the jagged ridges of the spines this ectodermal layer isoften so shallow as to be almost inappreciable.

The nucleus is large, and situated in the centre, or rathertowards one side of the cell; it is oval in shape, and has adense, deeply staining reticulum with several nucleoli.

In some regions of this ectodermal layer the cells are veryvacuolated; the nuclei of these cells are easily discernible,and are surrounded by a mass of coarsely granular protoplasm,from which protoplasmic strands extend to the cell borders(PL 1, fig. 5).

In other places, however, the cells are closely crowdedwith deeply staining roundish corpuscles, which are oftenpresent in such numbers as to obliterate from view theboundaries between the cells and hide the nuclei. Probablythe presence of these corpuscles is accounted for by the factthat the lower ectoderm apparently secretes some of the nema-tocysts which frequently occur in great abundance in theseparts of the lower ectoderm (PI. 1, fig. 1, c).

As was mentioned by Strethill-Wright (10), there are twokinds of nematocyst in Hydractinia, small and large; bothkinds are found in the lower ectoderm.

The ectoderm of the gasterozooid tentacles is crowded withthe small variety, while large nematocysts occur in the ecto-derm of the blastostyle and dactylozooid tentacles, and in theectoderm of the tips of the tentacular polyps; nematocystsare also found in small numbers throughout the ectoderm ofthe bodies of the gasterozooids, blastostyles, and dactylo-zooids, and in large numbers in the ectoderm of the bodies ofthe tentacular polyps, but I have never found both the largeand small variety occurring in the same individual.

B. His to logy of the Upper Ectoderm.1. Ectoderm of the Basal Ccenosarc.—This forms a

continuous and regular layer of cells, which are approximatelyequal in size, and appear cubical in section.

ON THE STRUCTURE OF HtDRAOTINlA JECHINATA. 93

These cells are apparently not characterised by muscle tails,but interstitial cells are sometimes wedged in between themat their bases (PL 1, fig. 5). There are no sensory cells inthis layer, but nematocysts are occasionally found. The nucleusis situated in the centre of the cell; it is round or slightly oval,and has always one large nucleolus: surrounding it is a massof finely granular protoplasm which almost fills the whole cell.

2. Ectoderm of the Polyps.—This layer is in manyrespects similar to the basal ectoderm; the cells are oftensomewhat narrower, more columnar in shape, and more variablein size.

They are characterised by exceedingly long muscle tails,which can be demonstrated by macerating a hydroid for aboutan hour in £ per cent, solution of acetic acid, washing in water,staining with picro-carmine for an hour or more, and examin-ing in weak glycerine.

The protoplasm of the muscle tails appears more homoge-neous aud less coarsely granular than that of the body of thecell. In several of these isolated ectoderm cells large vacuoleswere observed. (Process , Pig. 3.)

TIG. 3.

94 MARGARET C. COLLCUTT.

The muscle tails iu nou-macerated material are most easilydemonstrable in sections of dactylozooids, where they aregreatly developed (PL 1, fig. 8).

c. His to logy of the Mesogloea.

The mesogloea extends between ectoderm and endodermthroughout the colony as a structureless, homogeneous layer.In the basal part of the colony it is very thin, and, as men-tioned above, thin strands of its substance serve to fix thecoenosarc to the chitinous skeleton; in the polyps it formsa much thicker layer (PI. 1, fig. 1, c).

D. His to logy of the Endoderm.

1. Endoderm of the Basal Coenosarc.—The endodermlining the cavity of the ramifying tubes consists of a singlelayer of cubical cells, with large central oval nuclei. The proto-plasm is coarsely granular and abundant, and there are manyfood granules. Other deeply staining gland-like cells occurat intervals throughout the endoderm of these tubes (PI. 1,fig. 1, b); these are probably similar to certain so-called gland-cells which occur in the endoderm of the Gasterozooids.

2. Endoderm of the Gasterozooids.—This endodermallayer consists of long narrow cells, closely pressed against oneanother, and somewhat irregular in shape ; they vary in length,so that their free ends are not all on the same level. Theyare greatly vacuolated, especially towards their free ends,where they widen slightly. The nucleus is situated in themiddle region, and close to the border of the cell; it is oval,and has a granular reticulum, with one or more nucleoli.

In the hypostome the endoderm is thrown into longitudinalridges, which project into its cavity; there are usually fromfour to eight of these ridges, which then pass downwards andlose themselves in the endoderm of the stomach. A transversesection of the hypostome consequently represents the lumenof the gut as being star-shaped. Allman (14) mentions thatsimilar endodermal ridges occur in the endoderm of othergymnoblastic hydroids.

ON THE STKUOTDRE OF HYDBACTINIA EOHINATA. 95

In a gasterozooid which has had no food for some time theprotoplasm of the endoderm cells is exceedingly vacuolated,there being only a small mass of protoplasm congregated roundthe nucleus, and a thin protoplasmic cell lining; no otherendodermal elements can be demonstrated, at all events indead material.

On the other hand, an examination of the endoderm of agasterozooid which is digesting food clearly shows that thereis a certain amount of differentiation between the cells. Theordinary vacuolate endoderm cells are crowded with foodvacuoles; the nuclear plasm appears more granular than innon-digesting endoderm cells. But besides these vacuolatecells, there are numbers of pyriform cells which are situatedsingly between every two or three of the vacuolate cells, andwhich stain very readily. These cells are rather wider intheir middle region than the vacuolate cells, and taper offbasally where they come in contact with the mesogloea; thenucleus is situated towards the base of the cell and is sur-rounded by a dense mass of protoplasm. The rest of the cell iscrowded with small spherical bodies, which are so abundant asto hide the protoplasm in which they are embedded (PI. l,fig.9).

Miss Greenwood (16) describes similar cells in the endodermof Hydra under the name of " gland-cells." Among the cellcontents of the endoderm are numerous roundish bodies,smaller than nuclei, and especially abundant in the endodermof the hypostome; some of these stain equally throughout,but others only stain partially, a large slightly stainedregion being left. They stain deep blue with hsematoxylinbut yellow with borax carmine; they are colourless in theliving animal, and stain with iodine less than the other cellcontents. The most likely hypothesis seems to be that theyare proteid in nature; they may correspond to the " nutritivespheres" of Hydra described by Miss Greenwood (16). Endo-derm is continued into the tentacles as a single row of cellswhich are very vacuolated and regular in shape; the nucleusis central and surrounded by protoplasm, which forms a centralband in the cell.

96 MABGARET 0. OOLLOtfTT.

3. Endoderm of the Blastostyle.—The endodermlining the cavity of the blastostyle is mostly richly ciliatedwith very long cilia; the cells of the " head " are large, andeach contains a nucleus like that in the endoderm cells of therest of the colony; these cells digest food material, apparentlybecoming amoeboid at their free ends (PI. 1, fig. 6), the blasto-styles undoubtedly taking in nutritive material by theirmouths.

The cells of the " neck " are verylong, regular, and narrow,and there are numerous small nuclei, each containing severaldistinct nucleoli in this region, which is richly ciliated ; egg-cells are here present in abundance (PI. 1, fig. 6).

Below this region the body of the blastostyle dilates, andsporosacs arise; the cells here are smaller, have large nuclei,and cilia are not so apparent. Gland-cells are nowherepresent in the endoderm of blastostyles. The tentacles havea solid endodermai core as in the gasterozooids. Egg-cellshave been observed in the endoderm of the base of the blasto-style.

4. Eudoderm of the Dactylozooid.—This endoderm isvery vacuolated ; the cells are approximately equal in size, butare rather smaller towards the distal extremity of the polyp.The nuclei are situated in the middle of the cells. The endo-derra of the upper half of the body is often crowded with foodgranules, but I have never observed food masses in the lumenof the endodermai gut, nor are gland cells present.

5. Endoderm of the Tentacu la r Polyps.—This endo-derm is also crowded with food granules throughout its extent.The cells are very regular, long, and narrow, and the lumenof the gut widens considerably towards the base of the polyp.

ON THE STEUOTUEB OF HYDBA.CTINIA EOHINATA. 97

BIBLIOGRAPHY.

1. FLEMING, J.—"Alcjonium echinaturn,"'British Animals,'1828.2. JOHNSTON, G.—"Alcyonidium echinatum," 'British Zoophytes,'

p. 304, pi. xlii, figs. 3, 4,1838.3. JOHNSTON, G.—"Coryne squamosa, var.," 'British Zoophytes,'

pi. ii, figs. 4, 5,1838.4. VAN BENEDEN, P. J.—"Hydractinia, sp.,"'Bull, de l'Acad. Roy. de

Bruxelles,' torn, viii, 1841.6. HASSALL, A. H.—"Echinochorium clavigerum," 'Ann. Nat. Hist.,'

vol. vii, p. 371, pi. x, fig. 5, 1841.6. PHILIPPJ, A.—"Dysmorphosa oonchicola," ' Wiegman's Archiv,*

1842.

7. DE QUATREFAGES, A.—" Synhydra Parasites," ' Ann. des Sci. Nat./vol. xx, p. 230, pis. 8, 9,1843.

8. VAN BENEDEN, P. J.—"Hydractinia laotea and Hydract in iarosea,"'Rech. sur l'Embryoge'nie des Tubulaires, Mem de l'Acad.Roy. de Brux.,* torn, xviii, p. 104, pi. ix.

9. JOHNSTON, G.—"H. echinata," 'Brit. Zooph.,' p. 34, pi. 1, figs, 4, 5,1847.

10. STBETHILL-WEIGHT, T.—"Hydractinia echinata," 'Edinb. NewPhil. Journ.,' April, 1857.

11. VAN BENEDEN, P. J.—"H. echinata," 'Recherches sur la Faunelittorale de Belg.,' p. 134, pi. xl, figs. 1—8,1866.

12. HINCKS, T.—"H. echinata," ' Brit. Hydr. Zooph.,' p. 23, pi. iv.13. ALLMAN, G. J.—"H. echinata," ' Gymnoblastic Hydroids,' pp. 220,

342, pis. xv and xvi, figs. 10,11,1871.14. ALIMAN, G. J.—'Report on the Hydroida collected by H.M.S. 'Chal-

lenger,' p. ix, 1888.

15. WEISMANN, A.—' Enstehung der Sexualzellen bei den Hydromedusen,'Jena, 1883.

16. GBEENWOOD, M.—"Digestion in Hydra," 'Journal of Physiology,' vol.ix, 1888.

17. BUNTING, M.—" Origin of Sex-cells in Hydractinia," ' Journ. Morph.,'ix, 1894.

VOL. 4 0 , PAET 1.—NEW SBR.

98 MARGARET 0. COLLCUTT.

EXPLANATION OF PLATE 1,

Illustrating Margaret C. Collcutt's paper " On the Structureof Hydractinia echinata."

(Several different colonies were examined, hence there is a slight variation inthe size of cells, &c., figured.)

LIST OF REFERENCE LETTERS.

up.ect. Upper ectoderm. Led. Lower ectoderm, end. Endoderm. mes.Mesogloea. i. c. Interstitial cell. n. Nematocyst. n. c. Nematocyst cell.gl. Gland-cell, pip. Polyp, spn. Spinule. p. m. Perisarc membrane, c. t.Ccenosarc tube. p.t. Perisarc tube. e. Egg-cell, f.m. Food material.mo. Mouth, sk. Skeleton, v. Vaouole. m. i. Muscle tail, nies.p. Meso-gloea process, d. e. Degenerating ccenosarc. nu. Nucleus.

FIG. 1.—Series of vertical sections illustrating the transition from thegrowing edge to the older parts of the colony, a. Section at extreme edge,showing to the left a ccenosarc tube surrounded with perisarc, and towardsthe right tubes which have anastomosed. b. Section of somewhat olderpart of the colony, where the perisarc membrane tapers off and disappears.c. Section of old part of colony passing through portion of the wall of apolyp, and showing the communication between the polyp and an endodermaltube of the basal ccenosarc. The section also shows the tabulated growth ofthe chitinous skeleton. D. 3 cam.

FIG. 2.—Vertical section of base of colony, showing the degeneration ofccenosarc and a degenerating mass isolated in a cbitinous lacuna. D. 3 cam.

FIG. 3.—Diagrammatic section of a smooth spine at the colony edge.A. 3 cam.

FIG. 4.—Optical horizontal section of two anastomosing tubes, showingboth commencing anastomosis and complete anastomosis. The left-hand tubehas given off a branch. D. 3 cam.

FIG. 5.—Vertical section through a growing chambered spine illustratingthe relations of the basal ccenosarc to the skeleton, and showing a chamber ofthe spine containing ccenosarc. D. 3 cam.

FIG. 6.—Longitudinal section of upper part of blastostyle; on the right-hand side is an egg-cell, e', in the ectoderm, and two egg-cells in the endo-

ON THE STRUCTURE OF ffXDRAOTINIA BOHINATA. 99

derm of the " neck." Food is being digested by the endoderm cells of the" head." D. 3 cam.

FIG. 7.—Transverse section through a blastostjle showing an egg-cell,e", in the mesoglcea of the blastostyle, and four egg-cells in the endoderm ofthe sporosac. D. 3 cam.

FIG. 8.—Longitudinal section of a dactylozooid, showing food granules inthe endoderm of the upper part. D. 3 cam.

FIG. 9.—Longitudinal section of the endoderm of gasterozooid. D. 3 cam.

.i;. 4'ier: Sd. /v/. Wsffi.