VBBTBBBATA PISCES. 



81 



the heterocercal type, the vertebral column extends to 

 the extremity of the upper tail lobe. The greater por- 

 tion of the caudal fin is in 

 this case below the vertebral 

 column, the lobes being un- 

 equal. Most modern fishes 

 have homocercal tails, while 

 the heterocercal type is com- 

 monest in fossil fishes. The 

 sturgeons, sharks, &c., have 

 heterocercal tails. 



62. Digestion. Theteeth 

 of fishes are not confined to 

 the jaivs, but are usually 

 distributed over all the bones 

 which assist in forming the 

 cavity of the mouth They 

 are not inserted in sockets, 



being merely attached to the FIG. 18. TAIL FINS OP FISHES. 



Surface of the bones. The 1, homocercal tail; 2, heterocercal tail. 



gullet, which is short and wide, opens into a large stomach. 

 The inner extremity of the stomach, where it opens into 

 the intestine, is furnished with a valve, behind which are 

 a number of blind tubes, termed the "pyloric cseca," 

 which are thought to be the representatives of the pan- 

 creas. A true pancreas, however, is sometimes present. 

 The intestinal canal varies in length in different fish. 

 The liver is usually large, and contains much oil. The 

 kidneys are also so large as to .extend from the one 

 extremity of the abdomen to the other. 



63. Circulation. The heart generally consists of two 

 cavities, an auricle and a ventricle. The exceptions are 

 the Arnphioxus or Lancelet, which has no heart, and the 

 Lepidosiren or mud-fish, which has two auricles and one 

 ventricle. The venous blood, after traversing the body, 

 passes through the vena cava into the auricle which trans- 

 mits it to the ventricle. It passes from the ventricle into 

 the "branchial artery" which conveys it to the gills, 



