410 



ANATOMY OF VERTEBRATES. 



denticles and sieve-like plates or processes, fig. 85, 63, to prevent 

 the entry of food into the interspaces of the gills, and the branchial 



276 



Digestive organs in situ, Plauirostra. 



outlets are guarded by valves which reciprocally prevent the 

 regurgitation of the respiratory streams back into the mouth. 



The necessary cooperation of the jaws with the hyoid arch in 

 the rhythmical movements of respiration is incompatible with 

 protracted maxillary mastication ; and, accordingly, the branchial 

 apparatus renders a compensatory return by giving up, as it were, 

 the last pair of its arches to the completion of the work which the 

 proper or anterior jaws were compelled by their services to re- 

 spiration to leave unfinished : and thus the mouth of typical Fishes 

 is closed at both ends by dentigerous jaws. 1 



The first portal to the alimentary tract is usually formed by the 

 upper and lower jaws, fig. 276, a, b, and their teeth : the Gym- 

 nodonts are so called on account of their conspicuous manifestation 

 of this character, fig. 258. In most Fishes the jaws are covered 

 by the skin, which, in passing into the mouth, takes on the 

 character of the mucous membrane. In some Fishes the inteim- 

 ment is folded before passing over the jaws, and the arched 

 and fortified barrier is preceded by a fosse inclosed by fleshy lips. 

 The Wrasses {Labridce), Mullets {Mugilidaz), and the Carp-tribe 

 {Cyprinidaf) exemplify this character. In Ci'cnilabrus, Choerops, 

 and Julis, the lips are plicated. In Mugil labeosus the thick upper 

 lip has a transverse fold. In some Cyprinidce the labial organs 

 are developed to excess, as, for example, in the genus thence 

 termed Labeobarbus, in which the lips are not only unusually thick 

 ancT fleshy, but the lower one is produced downward like a pointed 

 beard : it forms a long cone in Mormyrus Petersii. The labiated 

 Fishes have not, however, so distinct a ' sphincter oris ' as 



1 The Mullets ' take in a quantity of sand and mud, and after having worked it 

 for some time between the pharyngeal bones, they eject the roughest and indigestible 

 portion of it.' clxxiv. iii. p. 411. 



