OCR Text |
Show 1905.] OF ELASMOBRANCH FISHES. 40 ancestral Vertebrates or for what particular mode of feeding the structure of their mouth was adapted. It is highly probable that some light might be thrown on the primitive use of the oral denticles, if one knew the precise nature and disposition of the skeletal structures which bounded the oral cavity in the primitive Vertebrates before the anterior branchial arches ceased to be purely gill-bearing and had acquired the special characters of jaws, as seen in the most primitive of existing Gnathostomata. In the primitive Vertebrata it is possible that the seizure, holding, or perhaps even the crushing of the food may have been effected by the movements of the ventral portions of the arches towards the roof of the oral cavity, after the fashion of the hypopharyn-geal teeth in connection with the hinder branchial arches in many Teleosts. If there be any truth in this suggestion, it will not be difficult to appreciate the physiological value of an extensive distribution of denticles over the greater part of the oral and pharyngeal mucous membrane in the primitive Vertebrates. With the evolution of special jaws at a later period, t he functional denticles would naturally tend to become restricted to them and constitute ordinary teeth, leaving, however, the residue of the stomodeal invasion of dermal denticles to become pharyngeal teeth, or gill-rakers, or to remain as vestigial structures, or to vanish altogether. E X P L AN A T IO N OF PLATE III. Tlie figures are all magnified about 80 times and are from preparations examined in glycerine, which, in the case of figs. 1-4, rendered them sufficiently transparent for the internal structure of the denticles to he seen. The spinous portions of the denticles are directed towards the caudal extremity of the fish. Reference Letters. £.jt>. = basal plate; d .t.= dentine tubuli; m.m.=niucous membrane; _p.<\ = pulp-cavity. Fig. 1. A piece of the lining from the floor of the oral cavity of Mustelus l&vis (p. 44), showing the very closely arranged denticles. Fig. 1 « . A single denticle from the same, viewed laterally. Fig. 2. A strip of the mucous membrane from the floor of the pharynx of Galeus canis (p. 44). The denticles are seen to have a regular and orderly arrangement, but are not so closely disposed together as in Mustelus. Fig. 2 a. A single denticle of Galeus, viewed laterally. Fig. 3. A portion of the mucous membrane lining the floor of the pharynx of Notidanus cinereus (p. 44). Fig. 4. A portion of the mucous membrane from the pharynx of Acantlnus vulgaris (p. 45), taken from where the last two branchial arches join the floor of the same. Fig. 5. A piece of the lining of the roof of the oral cavity of Ehina squatina (p. 45). Fig. 5 a. A single denticle from the same, viewed laterally. Fig. 6. A strip of the mucous membrane lining the roof of the pharynx of Sain clavata (p. 46), taken from near the last two branchial arches. In this species and in Rhina the denticles have lost their regular arrangement and are scattered over the mucous membrane as if at random. P r o c . Z o o l . Soc.-1905, V o l . I. No. I\ , 4 |