CHARACTERS OF FISHES. 397 



heart, but is driven from the branchiae through all parts of 

 the body ; the propulsive force necessary for this being derived 

 chiefly from the heart, assisted by the contractions of the 

 voluntary muscles. In some fishes (as in the Eel) the return 

 of the blood to the heart is assisted by a rhythmically con- 

 tractile dilatation of the caudal vein. The essential peculiarity, 

 then, of the circulation of fishes depends upon this that the 

 arterialised blood returned from the gills is propelled through 

 the systemic vessels of the body, without being sent back to 

 the heart. 



The Lancelet (Amphioxus}, alone of all Fishes, halTno 

 special heart, and the circulation is effected by contractile 

 dilatations developed upon several of the blood-vessels. In 

 the Mud-fish (Lepidosiren) the heart consists of two auricles 

 and a single ventricle. The blood-corpuscles of Fishes are 

 nucleated (fig. 159, e), and the blood is red in all except the 

 Amphioxus. 



As regards the digestive system of Fishes there is not much of 

 peculiar importance. The mouth is usually furnished with a 

 complicated series of teeth, which, in the Bony Fishes, are not 

 only developed upon the jaws proper, but are also situated 

 upon other bones which enter into the composition of the 

 buccal cavity (such as the palate, the pterygoids, vomer, 

 branchial arches, the glossohyal bone, &c.) The oesophagus 

 is usually short and capacious, and generally opens into a large 

 and well-marked stomach. The pyloric aperture of the stomach 

 is usually furnished with a valve, and behind it there is usually 

 a number (from one to sixty) of blind appendages, termed the 

 " pyloric caeca." These are believed to represent the pancreas, 

 but there may be a recognisable pancreas either alone or in 

 addition to the pyloric caeca. The intestinal canal is a longer 

 or shorter, more or less convoluted tube, the absorbing surface 

 of which, in certain fishes, is largely increased by a spiral 

 reduplicature of the mucous membrane, which winds like a 

 screw in close turns from the pylorus to the anus. The liver 

 is usually large, soft, and oily, and a gall-bladder is almost 

 universally present ; but in the Amphioxus the liver is doubt- 

 fully represented by a hollow sac-like organ. 



The kidneys of fishes are usually of great size, and form two 

 elongated organs, which are situated beneath the spine, and 

 extend along the whole length of the abdominal cavity. The 

 ureters often dilate, and form a species of bladder, the doubtful 

 representative of the allantois. 



Whilst the respiration of all fishes is truly aquatic, most of 

 them are, nevertheless, furnished with an organ which is 



