472 



ANATOMY OF VEETEBEATES. 



pericardium. In the Sturgeon the communication with the peri- 

 toneum is by a single ehjngated. caual extending along the ventral 

 surface of the ccsophagus. In the Planirostra and Chimseroids 

 the pericardio-peritoueal canal is^also single. In the Plagiostomes 



311 



TTcrirt and trill-archer, Per(^,^ XXIII. 



it bifurcates, after leaving the j^ericardium, into two canals, which 

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

 oesophagus : no ciliary movements have l^een 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 Mura^na, short fil)rous 

 bands supporting vessels pass from different parts of the peri- 

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

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

 sinus communicates with the auricle, and wliere the biillius 

 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 lavers of the posterior 

 aponeurotic partition between it and the abdomen. Tlie 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, ii. In the Plagiostomes. the 

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

 (L(i])hius), and the Sun-fish (Orth(/qorisais),ti\eoiiiiQ(!hy which the 

 great sinus communicates witli the auricle isgnardcd by twoscmihmar 

 valves; Init these are I'ar from being constant in the Telcostomi. 

 The auricle, when distended, ia larger in proportion to the -ventricle 



