FISHES CHAP. 
Los) 
bo 
(oe) 
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,’ 
the former receiving the 
venous blood from the 
sinus venosus, and the 
latter the aerated blood 
= from the lung-like air- 
_ bladder. 
Fic. 193.—Diagram of the structure of the heart in : 
different Fishes. A, In an Elasmobranch; B, in The sinus venosus and 
ee ; and C, in a Teleost. a, Auricle ; b.a, the auricle have very 
yulbus aortae; c.a, conus arteriosus; s.v, sinus 2 
venosus ; v, semi-lunar valves ; v’, auriculo-ventri- thin walls 5 the ventri- 
eee ; v.d, ventral aorta ; vt, ventricle. (From cular walls, on the con- 
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 
' There is an incomplete auricular septum in the Holocephali (e.g. Chimaera 
monstrosa), see Ray Lankester, Zrans. Zool. Soc. x. 1879, p. 502. 
