Tanks No. 10 and 21. 
69 
reveals, in advanced stages, the young fish with a bunch of external gills, 
at either side of the neck, and the large yolk attached by a hollow cord 
through which the food substance is passed 
into the alimentary tract. Gradually the 
provisional gill-threads disappear and the 
colouring of the body becomes more dis¬ 
tinct. When the yolk is consumed, the 
young Dog-fish is ready to hatch; it forces 
its way out through one end of the egg- 
case, where the two plates of which it is 
formed are not firmly united, and then 
swims about freely in the tank. Eggs and 
embryos are often seen in the Aquarium, 
as the Dog-fish breed freely in tank No. 10, 
and the fishermen often bring in branches 
of coral and other objects, to which the 
eggs are fixed. (These may be seen in tank 
No. 21). These young stages have of recent 
years become very important in scientific 
research, and zoologists have drawn largely 
upon the material which this Aquarium pro¬ 
vides. Economically, however, the Dog¬ 
fish is of very little use. Its flesh is of 
very indifferent quality and only eaten by 
the poor; the skin (shagreen) is used for 
polishing , and the liver yields a certain 
amount of oil. 
The Smooth-hound or Ray-toothed 
Shark, Mustelus (Fig. 119), is only rarely 
to be seen in the Aquarium. This is one 
of the most harmless of sharks; its blunt 
flat-topped teeth are not fitted for devour¬ 
ing anything but crabs and mollusks, which 
it captures at great depths. When placed 
in tank No. 10, it swims about at first in 
a very lively manner with most graceful 
movements of its glossy body, but it soon 
tires, and finally cannot lift itself from the 
bottom of the tank; nor will it touch any 
food in captivity. Its flesh is quite good 
eating. — Almost all sharks are viviparous, 
so that Scyllium is an exception to the 
rule. The Angel-fish or Squatina (Fig. 120), 
which forms a connecting link between the 
Sharks and the Rays, brings forth living 
young ones. It is an unsightly beast, always 
lying on the bottom of tank No. 10, and 
usually looked upon as dead by the visitors 
to the Aquarium. It is one of the most stupid and lazy fishes of the 
Fig. 119. Mustelus vulgaris , 
1 /6 nat. size. 
