DEVELOPMENT OF SHARKS AND RAYS. 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 cesophagus 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 head 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, asin higher Vertebrates, form- 
ing a common stem or chiasma, 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 
Chimera, 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,” expanding 
posteriorly into uterine chambers, which unite and open 
into the cloaca. (Huxley.) 
The sharks and skates are not prolific; having but few 
