ORGANS OF CIRCULATION. 239 



spleen is usually placed upon the right side, but often in the middle 

 line above or behind the stomach. 



The Peritoneum in Fishes invests the whole intestine, and is 

 attached above to the pericardium, so as to form a kind of diaphragm 

 or partition, though not one of a muscular texture ; it also com- 

 pletely clothes the sexual organs, but not the kidneys. In some 

 Fishes, as in the Plagiostomi, the Sturgeon, and Salmonidse, a pair 

 of openings are situated upon either side of the anus which lead into 

 the cavity of the peritoneum, and allow the ingress of water to the 

 cavity and its contained viscera. The intestine is seldom secured 

 l}y a perfect mesentery, as is the case in the lower Fishes, e. g. 

 Myxine, but this is usually effected only by some thin filaments or 

 vasiferous bands. It is remarkable that during the embryonic ex- 

 istence of all Fishes a mesentery appears to be fully formed, but dis- 

 appears at a later period by absorption. When a swimming-bladder 

 is present, it usually opens into the resophagus, but sometimes by a 

 second orifice into the stomach. 



In the genus Lepidosiren several peculiarities occur in the organs 

 of digestion, and both spleen and pancreatic glands are absent. 



ORGANS OF CIRCULATION. 



To give a general description of the organs of circulation in the 

 class of Fishes is attended with much difficulty, from the many pe- 

 culiar varieties of structure which they present in the several genera 

 and families. Still, however, we may attempt a general survey of 

 the vascular system by first selecting a normal example of the 

 Osseous Fishes, such as the Common Perch, and a Shark or Ray, as 

 types of the Chondropterygians. 



The principal or only heart in many Fishes is a branchial heart, 

 in other words, a heart that is placed between the trunks of the 

 branchial veins which it receives, and the trunks of the branchial 

 arteries, which it gives off; it corresponds therefore to the right 

 heart of Man and the higher Vertebrata, and is traversed by venous 

 blood alone. The heart consists of one auricle and one ventricle, 

 both of which are lodged within a pericardium, to the inner surface 

 of which the heart is frequently attached, as in many Amphibia, by 

 special filaments. In the Plagiostomi the pericardium communicates 

 by openings with the peritoneum, so that it is bathed by the water 

 brought through the apertures in that membrane, and already de- 

 scribed as being situated near the anus. The heart is placed be- 



