Circulatory System in Finfish and Shellfish

arteriosus. Inspite of containing four parts, the heart of a fish is 
considered two-chambered. 
The heart 

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Sinus venosus
: The first chamber is called the sinus 
venosus, it is the preliminary collecting chamber. In teleosts 
it is filled from two major veins called the hepatic veins and 
the left and right branches of the Curvierian ducts which in 
turn collect blood from the paired (left and right) lateral veins 
the inferior jugulars, the anterior cardinals and the posterior 
cardinals. However in the elasmobranchs only one hepatic 
vein leads into it.
 
Atrium
. From the sinus venosus the blood flows into the 
atrium. the atrium is the largest of the chambers and weakly 
muscular. It pushes the blood, with weak contractions in the 
ventricle.
 
Ventricle
. The ventricle is the only well muscled chamber, 
nearly as large as the atrium it is the work horse of the heart, 
its contractions drive the blood around the body.
 
 

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Bulbus arteriosus 
 
The last chamber of the fish heart is called the bulbus 
arteriosus in the teleosts, but the cornus arteriosus in the 
elasmobranchs. The difference between these chambers 
is that the cornus arteriosus of sharks and rays contains 
many valves while the bulbus arteriosus of bony fish 
contains none. Both are alike in being primarily elastic 
and work to reduce the pulsed nature of the blood leaving 
the ventricle giving it a more even, constant flow.

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