142 



PISCES. 



the moveable supporting bones of the upper and lower jaws 

 (Labroidea). The buccal cavity is distinguished by ite width, and 

 by the great number of teeth it contains, which are developed from 

 the papillae of the mucous membrane by dentinal ossification. There 

 are often two curved parallel rows of teeth on the upper jaw ; an 

 outer row on the premaxilla, and an inner row on the palatine, and 

 there may also be a median unpaired row on the vomer. On the 

 lower jaw there is only one curved row of teeth. There may also be 

 teeth on the hyoid arch and on the upper jaw (maxillae) and para- 

 sphenoid, and, as a rule, on the branchial arches also, especially on 

 the upper and lower pharyngeal bones. The teeth are distinguished 

 according to their shape into pointed conical prehensile teeth and 

 grinding teeth. 



A small, hardly moveable tongue is developed on the floor of the 

 Ch, jV at buccal cavity, and the 



lateral walls of the 

 pharynx are pierced 

 by the gill slits. Fol- 

 lowing the pharyn- 

 geal cavity, there is a 

 usually short, funnel- 



01 



FIG. 592.-Diagrammatic longitudinal section through the 



head of a larva of Petromyzon (after Balfour). N, nervous a large stomach, which 



system ; Ch, notochord ; Of, auditory vesicle (represented f rpnnpT ,ti v drawn 

 as visible) ; O, mouth ; Ve, velum ; H, thyroid involution ; lu v 



Ks, branchial pouches ; C, heart ; Ab, optic vesicle ; out into a CSecum of 



01, olfactory pit. considerable size (fig. 



591). Caacal appendages (pyloric appendages) are not unfrequently 

 met with at the entrance to the longer mid-gut (small intestine) 

 which is marked off by a valve ; they probably serve the purpose of 

 increasing the extent of the secreting surface of the alimentary canal. 

 The intestine is usually several times coiled, and its internal surface 

 is remarkable for the longitudinal folds of the mucous mem- 

 brane ; villi such as are found in the higher Vertebrates are only rarely 

 present ; but in the Selachians, Ganoids, and Dipnoi there is a peculiar 

 spirally-coiled longitudinal fold the so-called spiral valve which 

 contributes essentially to the enlargement of the absorbent surfaces. 

 A rectum is not always clearly marked off, and when present is 

 always short, and in the Selachians it is furnished with a csecal 

 appendage. The anus is usually situated far back, and is always 

 ventral and in front of the urinary and generative openings. In 

 fishes with jugular fins, and in some Teleosteans without ventral 



