326 THE DOG-FISH 



position of the heart and the fact that the blood is sent 

 directly from the heart to the respiratory organs are 

 characteristic vertebrate features : so also is the circumstance 

 that the blood from the stomach, intestine, &c., is taken by 

 a specially modified portion of the ventral vessel (portal 

 vein) through the liver on its way to the heart. The blood 

 is red, containing, in addition to leucocytes, oval corpuscles 

 coloured by haemoglobin (see p. 56). 



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

 at the posterior end of the dorsal region of the co3lome, 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 ccelome, 

 while the nephridiopores discharge not directly on the 

 exterior, but into a common tube. 



The gonads (ovaries, Ovy, 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 de- 

 veloped 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 (Br). It has the further peculiarity of being hollow, 

 a more or less cylindrical cavity, the neurocosle (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 



