330 THE DOG-FISH LESS. 



The excretory organs are a pair of kidneys (K) situated 

 at the posterior end of the dorsal region of the coelome, and 

 opening by ducts, the ureters (Ur\ into the cloaca. De- 

 velopment shows that they consist of an aggregation of 

 nephridia (Nph\ the nephrostomes of which open in the 

 young and sometimes throughout life, into the coelome, 

 while the nephridiopores discharge not directly on the 

 exterior, but into a common tube. 



The gonads (ovaries, Oi>y, or spermaries) are situated in 

 the anterior part of the ccelome, attached by peritoneum 

 to its dorsal wall. The sex-cells are differentiated from 

 ccelomic epithelium. The gonaducts of both sexes 

 (Ovd) are developed from the nephridial system of 

 the embryo. 



As already stated, the central nervous system is contained 

 in a cavity (C. Sp. Cav] of the dorsal body-wall, and is 

 therefore far removed from the ectoderm from which it 

 originates. It consists of a long cylindrical rod, the spinal 

 cord(Sp. Cd] which is continued in front into a complicated 

 brain (jBr). It has the further peculiarity of being hollow, 

 a more or less cylindrical cavity, the neuroccele (n. cce] ex- 

 tending through its whole length. 



The possession of a hollow nervous system lying altogether 

 dorsal to the enteric canal and ccelome, of either a noto- 

 chord or a chain of vertebral centra below the nervous 

 system, and of pharyngeal pouches communicating with the 

 exterior, are the three most characteristic features of the 

 vertebrate phylum. 



The organs of sense are highly developed, and consist of 

 paired olfactory sacs, eyes, and auditory sacs situated in the 

 head, together with an extensive system of integumentary 

 organs. Their sensory cells are in every case ectodermal. 



The eggs are very large, and are impregnated within the 



