SCYLLIUM CANICULA 245 



the animal via the carotid or systemic arteries and back to the heart 

 again. In spite of this double choice, however, the two blood 

 streams returning to the heart, the aerated and non-aerated blood, 

 are not kept absolutely separated in the ventricle, and thus we have 

 an incomplete double circulation. 



Urogenital System. 



The excretory and reproductive organs are conveniently 

 dealt with together, since they are closely related anatomically ; 

 indeed, the ducts of the reproductive organs are mainly derived 

 from the primitive excretory ducts. 



We may first consider the kidneys, since they are more 

 nearly alike in the two sexes, but in order to understand the condition 

 in the adult it is necessary to glance briefly at their development. 



The kidneys arise, as in all Craniates, as tubular organs closely 

 related to the myOtomes or muscle segments. The first of these to 

 develop are found in the early embryo towards the anterior end of 

 the body cavity in the form of three or four tubules on each side in 

 close proximity to the post-cardinal vein, and these constitute an 

 imperfect excretory organ known as the pronephros. On the internal 

 side each tubule possesses a ciliated funnel-shaped opening, the 

 nephrostome, opening into the ccelom. This leads into a tube, only 

 slightly coiled, lying in the thickness of the ccelomic wall, and a glome- 

 rulus such as we find in the kidney of Rana is either absent or only 

 imperfectly represented. At their outer end these tubules run into 

 a long duct, the pronephric or segmental duct, which passes in the 

 body wall back to the cloaca. In Scyllium the nephrostomes fuse 

 to form one opening. The pronephros is apparently never functional, 

 and is probably to be regarded as the remnants of a body functional 

 in ancestral forms. The tubules disappear in the adult, but the duct 

 in some form or other is always to be found. 



A short time after the pronephros makes its appearance, a second, 

 much longer series of tubules, about twenty-nine in number, arises 

 behind it. These tubules also have nephrostomes, but become more 

 convoluted, and although at first they end blindly in the body wall, 

 they soon acquire openings into the segmental duct. They constitute 

 the second excretory organ, the mesonephros or Wolffian body, which 

 soon acquires a duct for itself by a tube being split off from the 

 pronephric duct. The tubules themselves increase in length, become 

 highly convoluted and develop typical Malpighian bodies, so that 

 they form active excretory bodies. These tubules are often described 

 as the primary tubules in order to distinguish them from the secon- 

 dary tubules, of which two or three arise, by budding from each 

 primary tubule. The secondary tubules never develop nephrostomes, 



