506 PISCES— FISHES. 



may pass out by the gill-clefts ; the branchial chamber is 

 also washed by water which passes both in and out under 

 the operculum. The gill filaments borne on the four 

 anterior branchial arches are long triangular processes, 

 whose free ends form a double row. As there are no par- 

 titions between the five gill-clefts, the filaments project 

 freely into the cavity covered by the operculum. Along 

 each arch and filament there are blood vessels, bringing the 

 impure and removing the purified blood. On the internal 

 surface of the operculum lies a red patch, the pseudobranch 

 or rudimentary hyoidean gill. A large and quaint parasitic 

 copepod — Lerncea branchialis — is often found with its head 

 deeply buried in the tissues of the gills and head. Many 

 related forms are common on fishes. 



The swim-bladder lies along the dorsal wall of the 

 abdomen ; the duct which originally connected it with the 

 gut has been closed. The dorsal wall of the bladder is so 

 thin that the kidneys and vertebrae are seen through it ; the 

 ventral wall is thick, and bears anteriorly a large vascular 

 network or rele mirabile, which receives blood from the 

 mesenteric artery and returns blood to the portal vein. 



Circulatory system. — The heart lies within a pericardial 

 chamber, separated by a partition from the abdominal 

 cavity. The blood from the body and liver enters the heart 

 by the sinus venosus, passes into the thin-walled auricle, 

 and thence to the muscular ventricle. From the ventricle 

 it is driven up the ventral aorta, the base of which forms a 

 white non-contractile bulbus arteriosus. 



The ventral aorta gives off, on each side, four afferent 

 branchial vessels to the gills. Thence the blood is collected 

 by four efferent trunks, which unite on each side in an 

 epibranchial artery. The two epibranchials are united 

 posteriorly to form the dorsal aorta, while anteriorly they 

 give off the carotids, which are united by a transverse vessel 

 closing the " cephalic circle." 



Blood enters the sinus venosus by two vertical precaval 

 veins, and by hepatics from the liver. Each precaval vein is 

 formed by the union of a jugular from the head and a cardinal 

 from the body. The cardinals extend along the kidneys, 

 and are continuous posteriorly with the caudal vein, but the 

 middle part of the left cardinal is obliterated. 



