472 



ANATOMY OF VERTEBRATES. 



pericardium. In the Sturgeon the communication with the peri- 

 toneum is by a single elongated canal extending along the ventral 

 surface of the oesophagus. In the Planirostra and Chimaeroids 

 the pericardio-peritoneal canal is also single. In the Plagiostomcs 



311 



Heart and gill-arches, Perch, xxm. 



it bifurcates,, after leaving the pericardium, into two canals, which 

 diverge and open into the peritoneum, opposite the end of the 

 esophagus : no ciliary movements have been noticed on the 

 surface of these remarkable conduits. The serous layer of the 

 pericardium is defended by an outer aponeurotic coat in Osseous 

 Fishes and Plagiostomes, which adheres to the surrounding parts. 

 In the Sturgeon, Wolf-fish, Loach and Murrena, short fibrous 

 bands supporting vessels pass from different parts of the peri- 

 cardium to the surface of the heart : in most other fishes the heart 

 hangs freely except at the two opposite poles, viz. where the 

 sinus communicates with the auricle, and where the bulbus 

 arteriosus is continued into the branchial artery. 



In the Plagiostomes the sinus itself is situated within the peri- 

 cardium ; but in Osseous Fishes between the layers of the posterior 

 aponeurotic partition between it and the abdomen. The heart is sit- 

 uated below the hind-part of the gills, and, as these are more concen- 

 trated in the head in all Fishes above the Dermopteri,so the position 

 of the heart is more advanced, fig. 308, H. In the Plagiostomes, the 

 Sturgeons, and many Osseous Fishes, e.g. the Perch, the Angler 

 (Lophius), and the Sun-fish ( Orthagoriscus),the orifice by which the 

 great sinus communicates with the auricle is guarded by two semilunar 

 valves ; but these are far from being constant in the Teleostomi. 

 The auricle, when distended, is larger in proportion to the ventricle 



