DEVELOPMENT OF SHARKS AND RATS. 417 



nished with an upper and nether millstone for crushing and 

 comminuting the thick, solid shells of mollusks. The mouth 

 in both sharks and rays is always situated on the underside 

 of the head, all being ground-feeders. Such sharks as rise 

 to the surface for food seize it by turning over before closing 

 their jaws on the luckless victim. 



The throat or oesophagus is wide ; the stomach a capacious 

 sac, and the intestine short, separated from the stomach by 

 a pyloric valve. The spiral valve of the intestine is a fold 

 projecting into the cavity of the gut, the fixed edge, forming 

 a spiral line around the inner wall of the intestine. 



The heart consists of a ventricle and auricle, with an 

 aortic bulb which pulsates as regularly as the heart ; and the- 

 blood must be sent forward with great force, as the very mus- 

 cular bulb is provided within with three rows of semi-lunar 

 valves. 



The gills are pouch-like, generally five, rarely six or seven, 

 in number, the external openings or gill-slits being usually 

 of moderate size, but sometimes long and large, as in the 

 basking shark. While the clefts open on the side of the 

 neck in sharks, in the skates they are placed beneath the neck. 



A spiracle or opening leads, in some sharks, from the up- 

 per side of the liead into the mouth. According to Wyman 

 this is the remnant of the first visceral cleft of the embryo. 



In the brain the optic thalami are separate from the optic 

 lobes, the olfactory lobes being large and long in the skates 

 and some sharks. The medulla forms the larger part of the 

 brain. The optic nerves unite, as in higher Vertebrates, form- 

 ing a common stem or cldasma, before diverging to the eyes. 



The eyes of some sharks have a third lid or nictitating 

 membrane analogous to that of birds. The ear, except in 

 Gliimmra, has the labyrinth completely surrounded by carti- 

 lage. There are two testes, and usually two ovaries, but in 

 the dog-fishes and the nictitating sharks there is but a single 

 ovary. The oviducts are true " Fallopian tubes," expandmg 

 posteriorly into uterine chambers, which unite and open 

 into the cloaca. (Huxley.) 



The sharks and skates are not prolific ; having but few 



