Phylum Chordata 579 



arteries carry it to the dorsal aorta from which it is distributed by vari- 

 ous arteries to all parts of the body. A series of veins return the blood 

 to the heart. There is both a hepatic portal and a renal portal system. 

 The hepatic portal system carries the blood from the digestive tract 

 to the liver, while the renal portal carries it from the tail region to the 

 kidneys. 



For respiratory exchange, oxygen-laden water enters the mouth 

 and passes through the pharynx, past the gills and out through the 

 spiracles and gill slits. Gaseous exchange takes place through the 

 walls of the capillaries of the gill filaments. 



The two niesonephric kidneys are just above the peritoneum. Small 

 tubules from each join to form the urinary ducts which open in the 

 urogenital papilla which lies in the cloaca. 



The shark has a well-developed nervous system with a much more 

 highly developed brain than that possessed by the cyclostomes. The 

 olfactory lobes are large and well developed, the cerebrum has two small 

 cerebral hemispheres above ; there is a diencephalon with a pineal body 

 and an injundibulum, two optic lobes, a cerebellum, and a medulla. 

 Eleven pairs of cranial nerves supply the various sense organs and mus- 

 cles of the head region. From the spinal cord, paired nerves arise. 



The sense organs are comparatively well developed and include the 

 olfactory sac, the eyes, the ears, and the lateral line organs. In the head 

 region are a series of mucous canals which terminate in ampullae 

 at the end of the snout. Possibly these are also sensory in function. 



The sexes are separate, the male possesses two testes which are 

 in the anterior part of the body cavity. The vas efjerentia from each 

 testis forms a convoluted vas deferens which empties into the urogenital 

 papilla. The paired ovaries of the female are in the dorsal wall of the 

 abdominal cavity. The eggs pass from the ovaries and enter the 

 ostia of the oviducts. The anterior portion of each oviduct is enlarged 

 into a shell gland, and the posterior part into the uterus. Each oviduct 

 opens into the cloaca. The young are retained in the uterus during their 

 developmental period and are nourished by means of the large yolk 

 sac. 



Other Elasniobranchs. — Fossil remains indicate that during the 

 Silurian and Devonian periods, the elasmobranchs were extremely 

 abundant with the giant sharks dominating the seas. Even today, the 

 elasmobranchs have successfully exploited many of the different habitats 

 of the ocean. 



