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Show 48 MR. A. D. IMMS ON TIIE DENTICLES [Jan. 17, doubtedly placoid scales. I have omitted to give an account of their characters in the various species since they are described in detail for many forms by Steinhard, and I need only add that my own observations are in entire accordance with his results. 3. The facts which 1 have been able to make out lend but little support to the possibility of these structures performing any definite function. There appears to be no relation between the extent of the development and distribution of the denticles and the nature of the food of the various species in which they are found. For instance, in both Gale us cams and Mustelus Icevis the denticles are distributed over an exactly similar area, and there is but a small difference in the form of the individual denticles in the two cases. Nevertheless, Galeus preys on other fish, and has its teeth modified for that purpose, while in Mustelus the teeth are pavement-like, and are used for crushing the shellfish &c. on which it feeds. The spinous portions of the denticles were found in all cases to be directed towards the caudal extremity of the fish, and this renders it possible that the denticles may perhaps serve to roughen the mouth and, by this means, assist in the swallowing of the food. There is also the suggestion make by Steinhard, that they may serve to some extent in grinding up the food, but it is difficult to conceive that they could be of much utility in this direction, for in not a a few cases the denticles are so small as to only produce a barely perceptible roughness to the touch. It is possible that the denticles may subserve one or both of these functions, although their value in these respects must be very slight. A more probable view, and one more in accordance with their variable distribution and the absence of any obvious correlation between the nature of the food and the presence, absence, or degree of development of the denticles, is that these structures are vestigial organs. It is well known how tenaciously vestigial structures persist, even when they do not subserve any conceivable function, so long as their retention is harmless to the organism. In the case of the denticles, their persistence would not involve any serious tax on nutrition during their development, nor be detrimental in any other way, and under such circumstances, once they had been evolved for any special purpose, the tendency of heredity might be sufficient to secure their retention, even though their primitive physiological value had become lost. The fact that the denticles are relatively late in developing argues strongly in favour of their being vestigial organs. Thus in an Acanthias vulgaris 26 cm. long, although the teeth and dermal denticles were present, oral and pharyngeal denticles had only commenced to develop over a very limited area. In a Carcharias glaucus 39 cm. long these denticles had not yet attained their full development; and in a Pristiurus melano-stomus 14 cm. long no indications of them were to be detected. Unfortunately we know nothing concerning the habits of the |
