328 



FISHES 



two cavities are connected by a single pericardio-peritoneal canal, 

 or by two such canals ; and in Chimaera, and in the Sturgeon 

 (Acipenser) and Polyodon, by a single canal. 



The heart consists of at least three chambers, a sinus venosus 

 which receives the venous blood from the body, an auricle and 

 a ventricle, to which is added a conus arteriosus in the Elasmo- 

 branchs, certain Teleostomi (Crossopterygii, Chondrostei, and 

 Holostei), and in the Dipnoi. Through these cardiac chambers 



the blood is forced in 

 the order mentioned. In 

 the Dipnoi the auricle is 

 subdivided by a more 

 or less complete inter- 

 auricular septum into a 

 right and left auricle, 1 

 the former receiving the 

 venous blood from the 

 sinus venosus, and the 

 latter the aerated blood 

 from the lung-like air- 



\\\ Q r] rj AT* 



FIG. 193. Diagram of the structure of the heart in ' . 



different Fishes. A, In an Elasmobrauch ; B, in ihe SinUS VenOSUS and 



Amia ; and C, in a Teleost. a, Auricle; b.a, ^g auricle have Very 

 bulbus aortae ; c.a, conus arteriosus ; s.v, sinus . 



venosus ; v, semi-lunar valves ; v', auriculo-ventri- thin Walls J the Ventri- 



cular valve ; v.a, ventral aorta; t* ventricle. (From cukr wallgj Qn the CQn . 



trary, are very thick and 



in great measure are composed of a sponge-like network of mus- 

 cular bundles which generally encroaches considerably on the ven- 

 tricular cavity. Membranous valves, the sinu- auricular, and the 

 auriculo- ventricular valves, are developed at the junctions of the 

 sinus venosus with the auricle, and the auricle with the ventricle 

 respectively. The conus arteriosus is muscular and contractile, 

 and is interposed between the ventricle and the root of the 

 ventral aorta. Internally, the conus is provided with several 

 transverse rows of pocket -shaped or semilunar valves. In 

 Teleosts the conus is non-muscular and vestigial, and has but 

 a single row of valves, corresponding to the most anterior 

 of the multiple rows of valves in the Elasmobranchs. In these 



1 There is an incomplete auricular septum in the Holocephali (e.g. Chimaera 

 monstrosa), see Ray Lankester, Trans. Zool. Soc. x. 1879, p. 502. 



