SCYLLIUM 55 



derived from myotomes are striated, voluntary, and innervated 

 by ventral nerve-roots. The muscles of the jaws and branchial 

 arches, although visceral, are striated and voluntary ; they are, 

 however, not innervated by ventral roots, but are supplied by 

 dorsal cranial nerves. The remainder of the visceral muscles 

 are all smooth and involuntary, and are to be found in the walls 

 of the gut, blood-vessels, and oviducts. They are innervated 

 by the autonomic system (sympathetic and parasympathetic). 



Urino-genital System. — The kidney of the adult dogfish 

 is a mesonephros, similar to that of Petromyzon. Here, 

 however, the excretory and genital systems are closely associated, 

 and it is necessary to treat them together. In Scyllium and all 

 Gnathostomes, in place of the single mesonephric duct on each 

 side, there are typically two. One of these, the Wolffian duct, 

 can be regarded as the original mesonephric duct, and it con- 

 tinues to receive the tubules from the Bowman's capsules. 

 The other is the Mullerian duct which opens into the coelom 

 by the conjoined openings of the degenerated pronephric 

 tubules, and leads straight back to the cloaca without any 

 connexion with the mesonephric tubules. The degree of 

 development which these ducts show depends on the sex of 

 the animal. 



In the male, the testes are paired, and are connected by 

 their anterior ends to the mesonephric tubules by means of 

 the vasa efferentia. These correspond to the original 

 ccelomostomes. Through them the sperms reach the 

 Wolffian duct, which becomes known as the vas deferens ; 

 its posterior end swells to form the seminal vesicle. The 

 anterior portion of the mesonephros therefore is concerned 

 with the evacuation of the genital products in the male. The 

 posterior portion (sometimes and incorrectly called the meta- 

 nephros) is solely excretory in function. Its tubules run into a 

 collecting duct which connects with the Wolffian duct, both 

 running into a sperm-sac. The two sperm-sacs, one on each 

 side, join to form a urino-genital sinus which opens into the 

 cloaca by a urino-genital papilla. The Mullerian ducts in the 

 male are reduced to a pair of funnels on the ventral side of the 

 oesophagus, and the sperm-sacs. 



