26
ON THE TEETH OP PETLIOMYZON AND MYXINE. 631 On the Teeth of Petromyzon and Myxine. By Ernest Warren, D.Sc, Assistant Professor of Zoology, University College, London. With Plate 34. THOSE observers who regard the Cyclostomata as a degene- rate offshoot from a gnathostome ancestor would naturally desire to look upon the horny teeth as degenerate structures, and they would endeavour to find in them remains of parts homologous to those of an ordinary vertebrate tooth. In 1889 Dr. Beard 1 published an investigation on the teeth of the Marsipobranchii. The teeth of Bdellostoma, Myxine, and several species of Petromyzon were examined, and the results arrived at seemed conclusive as to the degenerate nature of the teeth. In the young tooth of the Hag-fishes a more or less complete enamel epithelium, with perhaps a trace of enamel, was identified; also semi-calcified odontoblasts were described, forming a conical mass beneath the enamel epithelium. More recent observers, however, have thrown doubts on these interpretations. In 1894 Prof. Howes 2 expressed his opinion that the " odontoblasts" exhibited no calcification, and in the same year Ayers 3 stated that he was unable to find a trace of enamel or dentine. 1 "The Nature of the Teeth of the Marsipobranch Fishes," ' Zoologische Jahrbiicher,' iii, 1889. A bibliography is there given. 2 'Nature,' Nov., 1894, Review of the Wood's Holl Lectures, s Biological Lectures at Wood's Holl, 1894,

Typhlorhynchus nanus: a New Rhabdocoele

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TYPHLOJIHYNOHUS NANCJS. 637

Typhlorhynchus nanus: a New Rhabdocoele.

By

F. F. Laidlaw, B.A.

With Plate 35.

THE small Rhabdocoele described below was found by Mr.Goodrich at Naples, living on the body of the Polychasteworm Nephthys scolopendroides, Delle Chiaje. He senta number of specimens preserved with Lang's reagent toDr. Gamble, who was good enough to hand them over to mefor examination. I am indebted to Mr. Goodrich for afurther series of specimens, some fixed with corrosive sub-limate and acetic acid, others with Lang's reagent; also forfigs. 2, 6 a, b, drawn from life. I have, unfortunately, beenunable to study living specimens; hence my account, especiallyas regards the genital organs, is necessarily somewhat incom-plete. The work was done in the Zoological Laboratory atOwens College.

The external appearance and genera l charac te rsare shown in PI. 35, fig. 1.

The total length of the body varies from *7 mm. to 1 mm.In size, therefore, it is quite comparable to a large infusoriansuch as Stentor polymorphus, Mull. The variations inlength between different specimens depend largely on theamount of extension or contraction of the body, especially ofthat part of it constricted off to form the snout or proboscis.

The body is spindle-shaped (see PI. 35, fig. 1), the anteriorend more pointed than the posterior. The front fifth of the

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