INTRODUCTORY 245 



line on the side of the fish. The sense-organs are innervated 

 by a branch of the tenth cranial nerve called the vagus, this 

 branch running parallel to the tube and sending a small branch 

 to each sense-organ. On the head the structures of this system 

 are not so constant : in Teleosteans they form tubes like that 

 of the lateral line, one running above the eye, another below 

 the eye and along the upper jaw, and a third along the front 

 of the operculum and the lower jaw ; these tubes are all sup- 

 plied by branches of the seventh cranial nerve. In the dog-fish, 

 on the other hand, and in most Elasmobranchs, the tubes on the 

 head are represented by separate tubes each opening by a pore 

 and running obliquely beneath the surface of the skin ; these 

 sensory tubes are situated in the same regions as the contin- 

 uous tubes of Teleosts, and are supplied by the same nerves. 

 It will be noted that the series of lateral-line organs radiate from 

 a focus formed by the auditory organ which is really to be re- 

 garded as a special and enlarged sense-organ of the same system. 

 The primary reproductive organs or gonads are outgrowths 

 of the epithelium lining the body cavity. In Elasmobranchs 

 the ovary has no connection with the oviducts ; the eggs, as in 

 the majority of vertebrates, are set free in the body cavity and 

 make their way into the open internal ends of the ducts ; the 

 latter are provided in the oviparous species with glands which 

 secrete a flexible, tough egg-shell of oblong shape. In the male 

 the testes are connected by fine efferent ducts with the epi- 

 didymis, originally a part of the kidney system, while the func- 

 tional kidney situated behind the epididymis has ducts of its own. 

 In the bony fishes the original oviducts degenerate and become 

 shorter and in the most modified Teleostei, such as the Perch or 

 Cod, the mouth of the duct has become continuous with a sac 

 which encloses the ovary so that the eggs do not become free in 

 the body cavity ; in the bony fishes also there is no shell-gland 

 and therefore no egg-shell, and the eggs when shed are 

 enclosed only in the vitelline membrane formed in the ovary. 

 In the male bony fishes the efferent ducts of the testis have 

 become a single duct which has separated from the urinary 

 duct and opens directly to the exterior. In the Elasmo- 

 branchs the intestine, the generative ducts, and the urinary 

 ducts all open into a common sac, the cloaca, which has a 

 single opening to the exterior as in birds and reptiles ; in the 



