CLASS CHONDRICHTHYES. CARTILAGINOUS FISHES 



375 



Pinea! body- 

 Lateral ventricfe 

 Cerebrum- 



Optic !obe 



Cerebetlum 



—Fourth ventricfe 



Optic chiosma 

 Third ventricle 

 Infundibulum 



Medulla oblongata 



Aqueduct of Sylvius 



Pituitary 



Figure 249. Longitudinal section between the first and second ventricles of the brain of the 



dogfish shark showing its structure. 



to the surface. Inside the canal are sensory 

 hair cells connected to a branch of the 

 tenth cranial nerve. On the surface of the 

 head are also sensory canals, which open 

 into pores containing pit organs with sensory 

 hairs. 



each oviduct is called a shell gland, and a 

 posterior part is enlarged in the dogfish to 

 form a "uterus" in which the young de- 

 velop (Fig. 250). The oviducts have separate 

 openings into the cloaca. 



Urogenital system 



The dogfish shark possesses two ribbon- 

 like kidneys (Fig. 250), one on either side 

 of the dorsal aorta. Their secretion is car- 

 ried by small ducts into a larger one, the 

 urinary ( mesonephric ) duct which empties 

 into a urogenital sinus; it then passes out of 

 the body through the cloacal opening. A 

 series of yellowish bodies called adrenals 

 are located on the medial border of the 

 kidney. 



The sexes are separate. The sperms of the 

 male arise in two testes and are carried by 

 the vasa eflFerentia to the much convoluted 

 vasa deferentia which empty into the uro- 

 genital sinus. During copulation the sperms 

 are transferred to the oviducts of the female 

 with the aid of the claspers. 



The eggs of the female arise in the paired 

 ovaries, which are attached to the dorsal 

 wall of the abdominal cavity. They break 

 out into this cavity and enter the funnel- 

 like opening, the ostium, common to both 

 oviducts. An expanded anterior portion of 



OTHER CHONDRICHTHYES 



The cartilaginous fishes now living are all 

 that remain of a type that once dominated 

 the ancient seas. Most of them occur in the 

 warm waters of the tropics. The sharks are 

 the largest of all fishes; they are to fishes 

 what elephants are to land animals. The 

 whale shark {Rhineodon typicus) is from 

 40 to 50 feet long. The sharks are the larg- 

 est living vertebrates with the exception of 

 the whales. Among the interesting species 

 is the great white shark, Carcharodon car- 

 charias, which reaches a length of 36Vi feet 

 and has earned the name of man-eater by 

 occasionally devouring a human being. One 

 of the most peculiar sharks is the hammer- 

 head (Fig. 251); its head is shaped like the 

 head of a mallet, with an eye on each side. 

 The sawfish (Fig. 251) is abundant in the 

 Gulf of Mexico and reaches a length of from 

 10 to 20 feet. The saw is about 5 feet long; 

 it is used for the capture of its prey; it 

 swings the "saw" back and forth in a school 

 of fishes to injure some of them sufficiently 



