OCR Text |
Show 306 PEOF. T. W. BEIDGE ON THE [Apr. 2, and its functional relations to other parts of the mechanism, must not, however, be left out of consideration. This organ (fig. 5) is an elongated band-like structure, somewhat broader behind than in front, and consists of a continuous plate of tooth-bone confluent below with the upper surfaces of the large basihyal and the two most anterior of the basibranchial elements. The tooth-bone carries over the whole extent of its upper or oral surface an extensive series of closely-set teeth, the largest of which are arranged in three parallel longitudinal rows-a lateral series (l.t.) along each * outer margin of the " tongue," and a mesial row (m.t.) along the centre. The lateral teeth, although somewhat smaller, are very similar in shape and disposition to the linear series in each mesopterygoid, and, like the latter, diminish in size from behind forwards ; the central teeth, on the contrary, are smaller in size, obtusely conical in shape, and much less regularly arranged. Between the three principal series of " lingual " teeth the surface of the " tongue " has a granulated appearance from the presence of a number of minute, conical, closely-set teeth. In its natural position in the mouth the linear series of lateral teeth are vertically opposable to the corresponding series of mesopterygoid teeth, while the mesial series lie between and below the latter. It is probable, therefore, that all three series of teeth (viz. the mesopterygoid, the parasphenoidal, and the "lingual") co-operate in effecting the mastication of the food-the mesopterygoid teeth by means of their lateral motion in the horizontal plane, and the lingual teeth through their vertical movements, or possibly, but improbably, by a kind of rasping action. The capacity for lateral movement on the part of certain of the oral bones is by no means peculiar to Osteoglossum. In the Pike (Esox), for example, and probably also in many other Teleosts, there can be no doubt that the proximal elements of the hyoid and mandibular arches and the various bones of the palato-pterygoid series are capable of similar movements as a more or less rigid whole, and that, in consequence, the jaw-system is similarly capable of lateral expansion and contraction. The capacity of the bones in question for lateral movement is no doubt of great service in widening the gape and thereby adapting tbe jaws for seizing relatively large prey, or it m a y be of considerable advantage in enabling the fish to firmly retain its possibly struggling prey in the oral cavity prior to the act of swallowing, or even in effectually aiding deglutition itself; but, at the same time, it is extremely doubtful if anything of the nature of oral mastication can be effected by these means. On the other hand, in none of the ordinary Teleosts is there any articulation between the metapterygoid and the base of the skull, and whatever lateral mobility the bones under discussion possess is entirely due to the nature of the hyomandibular-pterotic and the ethmo-palatine articulations. The special peculiarity of Osteoglossum lies in the fact that, while the jaws are capable of the same kind of lateral movement as in the Pike, there is, in addition, a secondary |