18 MORPHOLOGY OF THE VERTEBRATA. 



bladder are derived from the vagus and sympathetic, and its 

 function is a mechanical one, regulating the specific gravity of the 

 cod, and aiding it to maintain a particular level in the water. 



THE KIDNEYS 



Form two elongated reddish-brown organs, which are situated 

 beneath the spinal column above the air-bladder, and extend the 

 whole length of the abdominal cavity, from the head in front to 

 the tail behind. They are divided, for descriptive purposes, into 

 three parts — the fore, hind, and middle kid ney. They are broadest 

 and thickest anteriorly. The renal tissue is soft and spongy, and 

 is supplied by numerous small arteries from the abdominal aorta. 

 They receive a great part of their blood from the caudal vein, 

 which ramifies in them. 



The ureter passes directly downward, and dilates interiorly 

 into a bladder, which lies behind the rectum, and consists of two 

 portions, marked by a constriction. The bladder should be care- 

 fully blown out through the urethra. The urethra continued 

 from the bladder is a short tube, which opens behind the genital 

 apparatus. 



GENERATIVE ORGANS. 



Male — The testes are remarkable for their enormous increase 

 at the breeding season. When fully developed, they are commonly 

 known as the " soft roe." The greater part of their substance is 

 composed of the tubuli seminiferi, which open at one end into 

 the wide commencement of the vas deferens, and terminate at 

 the other by blind extremities. 



Female — The ovaries, popularly known as the " hard roe," form 

 two lengthened sacs, with a thin fibrous investment and a peri- 

 toneal covering, being closed in front, but produced behind into 

 a short and wide oviduct, which coalesces with its fellow of the 

 opposite side into a single tube — the common terminal portion 

 being much dilated, and terminating behind the anus in front 

 of the urethra. The stroma, or that part of the ovary which is 

 the seat of development of the ova, is extended by being plaited 

 into numerous transverse folds. 



CIRCULATORY SYSTEM. 



The heart has the typical piscine structure. It consists of a 

 sinus venosus, pouring its blood into the single auricle, and a 

 single ventricle receiving blood from the auricle. The ventricle 

 is continued into thQm^^tyfflmQMch it is separated by a 



