xm PHYLUM CHORDATA 227 



instances occur in which there is a marked differentiation of the 



teeth, those in the front of the jaws (Fig. 902) being pointed or 



chisel-edged, and adapted for seizing, while the back teeth have 



spherical surfaces adapted for crushing. In the Wrasses (Fig. 890, B) 



strong crushing teeth are developed 



on the pharyngeal bones. In the 



Globe-fishes the teeth are apparently 



reduced to one or two in each jaw, 



but each " tooth " in this case really 



consists of numerous calcified plates 



fused together. The teeth may be 



either simply embedded in the mucous 



membrane so as to be detached when 



the bones are macerated or boiled, or 



they may be implanted in sockets of 



the bone, or ankylosed to it. They 



are formed of some variety of dentine, 



and are often capped with enamel. Their succession is perpetual, 



i.e. injured or worn-out teeth are replaced at all ages. 



In some species the enteric canal shows little differentiation into 

 regions, but, as a rule, gullet, stomach, duodenum, ileuin, and 

 rectum are more or less clearly distinguishable. The stomach is 

 generally V-shaped, but its cardiac region may be prolonged into 

 a blind pouch ; it is often very distensible, allowing some of the 

 deep-sea Teleostei to swallow Fishes as large as themselves. In the 

 Globe-fishes the animal can inflate the gullet with air, when it floats 

 upside down on the surface of the water. The Ganoids have a spiral 

 valve in the intestine, which is very well developed in Polypterus 

 and the Sturgeon, vestigial in Lepidosteus (Fig. 904, sp. v.) and 

 Amia : it is absent in all Teleostei, except possibly in Chirocenlrus, 

 one of the Physostomi. The liver is usually large ; a pancreas 

 may be present as a compact gland, as in Elasrnobranchs, or may 

 be widely diffused between the layers of the mesentery, or in part 

 surrounded by the liver. Pyloric cceca are commonly present, and 

 vary in number from a single one to two hundred. The anus is 

 alwavs distinct from, and in front of, the urinogenital aperture. 



Respiratory Organs. The gills are usually comb-like, as in 

 the Trout, the branchial filaments being free, owing to the atrophy 

 of the interbranchial septa. In the Sturgeon, however, the septa 

 are fairly well developed, reaching halfway up the filaments, so 

 that the latter are free only in their distal portions ; this arrange- 

 ment is obviously intermediate between the Elasmobranch and 

 Teleostean conditions. The most striking deviation from the 

 normal structure occurs in Lophobranchii, in which the gill- 

 filaments are replaced by curious tufted processes (Fig. 892, B, </.). 

 As a rule gills (holobranchs) are developed on the first four branchial 

 arches, but the fourth is frequently reduced to a hemibrauch, 



