116 VERTEBRATE ZOOLOGY 



is short but of large diameter and has a secreting surface greatly en- 

 larged by a fold, in the shape of a spiral staircase, called the spiral 

 valve (a primitive fish character). Into the intestine empties the 

 large bilobed liver which is provided with a gall-bladder and a bile-duct. 

 A diffuse pancreas also pours its secretion into the intestine. 



The Respiratory System. Branchial respiration is carried on 

 in the six pairs of branchial clefts. These branchiae are primitive res- 

 piratory organs consisting of mere diverticula of mucous membrane, 

 richly vascular and supported by cartilaginous processes, called gill- 

 rays. The water enters the mouth and is forced out through the gill- 

 slits. In doing so, it aerates the gill-filaments and provides oxygen 

 for the blood that circulates rapidly through them. 



The Circulatory System (Fig. 63). The architecture of this 

 system is in the main laid out in accord with the branchial system. 

 The heart receives only venous blood from a single precaval vein, 

 and pumps it forward in a common ventral aorta, which divides into 

 five pairs of afferent branchial arteries, each of which carries blood to 

 one set of branchiae. A corresponding efferent branchial vessel picks 

 up the aerated blood from each gill and carries it to a dorsal aorta 

 which in turn distributes the blood to all parts of the body both an- 

 terior^ and posteriorly. A complex system of veins consisting of a 

 general systemic part, a hepatic portal, and a renal portal system, re- 

 turns the blood to the heart along paired channels called anterior 

 and posterior cardinal veins. 



Urogenital System. The nephridia (kidneys) consist of paired 

 strap-like organs lying side by side along the roof of the body cavity. 

 Microscopic examination shows that these long organs are composed 

 of paired nephric tubules each of which opens at one end into the cce- 

 lom and at the other into a nephric duct that leads to the cloaca. The 

 functional adult kidney is a mesonephros or " mid-kidney," the pro- 

 nephros being reduced to a mere vestige, though functional in the 

 larva. The testes are paired whitish bodies of rather flat, long and 

 narrow shape, that communicate with the cloaca by paired ducts, 

 the vasa deferentia. The ovaries are, when mature, large lobulated 

 bodies attached to the dorsal anterior part of the body cavity and 

 communicating with the exterior by large paired oviducts, which 

 unite anteriorly and have a single funnel-like opening- for receiving 

 the large ova. Each oviduct is regionally modified into a shell gland 

 and a uterus. 



