OCR Text |
Show 1905.] OX THE DENTICLES OF ELASMOBRANCH FISHES. 41 one instance 10 mm., of which the proboscis forms only the fifth part, i. e. 2 mm. The ventral hooks lie close up behind the proboscis. The skin is rather thin, and only partially transparent so far as concerns most of the internal organs, but the nerve-cord is clearly visible from the outside. The structure of the papillate bodies is shown in fig. 5. E X P L AN A T IO N OF PLATE II. Fig. 1. Physcosoma socium (p. 37). a. Dissection showing internal anatomy. b. Papilla from mid-body. c. Skin of introvert, showing gradual flattening of the papillae. Fig. 2. Physcosoma gaudens (p. 38). Papilla from the hind end of the borly. Fig. 3. Phascolosoma pyriformis (p. 39). a. Dissection showing internal anatomy. b. Papilla from the front end of the body, surface view. c. The same in section. Fig. 4. Aspidosiphon insularis (p. 40). a. Dissection showing internal anatomy. b. Hooks, c. Papilla from the base of the introvert. Fig. 5. Thalassema sabinum (p. 40). Skin from the front of the body. 7. On the Oral and Pharyngeal Denticles of Elasmobranch Fishes*. By A. D. Im m s , B.Sc. (Lond.), Zoological Laboratory, University of Birmingham. [Received November 1, 1904.] (Plate I ll.t ) .It is well known that in the Elasmobranch Fishes true teeth are carried only in relation with the palato-quadrate and mandibular cartilages. Minute denticles, however, may be present in greater or less abundance in many parts of the lining of both the oral and pharyngeal cavities. Very little has been written with regard to these structures, and, although reference is made to them by Hertwig, Popta, and others, the only general description of them is that recently published by SteinhardJ. I have been led to devote some attention to them as the outcome of an account which I have recently given of the structure of the gill-rakers of the Ganoid Fish Polyodon spathula §, In that paper I suggested that the gill-rakers of Polyodon may perhaps be regarded as scales (or denticles) which have migrated from the exterior of the body on to the branchial arches, and have there become greatly modified into long setiform structures. In order further to test the possibility of this suggestion, I have examined examples of species belonging to a considerable number of genera of Elasmobranchs for the purpose of ascertaining whether denticles of any description are present on the branchial arches in those Fishes. Given the presence of denticles on the branchial arches in such forms, it would not be difficult to conceive that the type of * Communicated bj' Prof. T. W . B s i d g e , F.R.S., F.Z.S. f For explanation of the Plate, see p. 49. t Archiv fiir Naturgesch. lxix. Bd. i. 1903, pp. 1-46, Taf. i. & n. § Proc. Zool. Soc. 1901, vol. ii. pp. 22-35, pi. ii. |