474 MANUAL OF ZOOLOGY. 



aerated blood which has passed through the gills is not re- 

 turned to the 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 contrac- 

 ; tions of the voluntary muscles. In some fishes (as in the Eel) 

 the return of the blood to the heart is assisted by a rhythmically 

 contractile dilatation of the caudal vein. The essential peculi- 

 arity, 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 

 iback to the heart. 



"""The Lancelet (Amphioxus}^ alone of all fishes, has no 

 special heart, and the circulation is effected by contractile 

 dilatations developed upon several of the blood-vessels. In 

 the Mud-fishes (Ltpidosiren) the heart consists of two auricles 

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

 nucleated (fig. 245, 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 may be also situ- 

 ated 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 cseca. The intestinal canal 

 is a longer or shorter, more or less convoluted tube, the ab- 

 sorbing 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 

 doubtfully 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 doubt- 

 ful representative of the allantois. 



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



