104 



Part second. 



municate with the mouth, forming a "hare-hp", a condition 

 passed through the embryos of all higher Vertebrates. These 

 external characteristics alone would distinguish the Sharks from 

 the bony fishes, but there are besides many differences of internal 

 structure. The skeleton of the Shark, Ray, or Dog-fish, is all 

 gristle, there being no bone except in the spines of the skin ; where 

 this bends over the jaws, they are enlarged to form the teeth. 

 The anatomy in general is less advanced in specialization than 

 that of the bony fishes. The commonest genus is the Spotted 

 Dog-fish, Scyllium, of which two species exist in European seas, 

 S. catulus, the Rock Dog-fish, or Bounce, as it is frequently called, 

 and S. canicula, the Little Dog-fish or Morgay. The first is about 

 a yard, the latter about two feet in length, so that they both 

 belong to the smaller kinds. They are lazy fishes, which seek 

 their prey usually at night-time, and lie hidden in the corners of 

 the tank during the day. They are fed on dead fish, which in day- 

 light they find chiefly by their sense of smell, as their eyes are 

 then quite useless; they search about close to the bottom of the 

 tank and only notice their food when they touch it with their 

 snout. In greed they are not far behind their relations the large 

 Sharks and can manage huge mouthfuls with their large well- 

 toothed jaws. They are, however, able to fast for considerable 

 lengths of time and some Scyllium catulus wihch were given nothing 

 to eat for months showed no signs of weakness. 



The Dog-fishes pair in the Aquarium. The female lays its 

 eggs singly on branches of coral {e. g. tank Nr. lo on the right) 

 or attaches them to bushes or rocks. They are contained in 

 rectangular, leathery capsules known as Mermaid's or Sailor's 

 purses, semi-transparent and white when first laid, but after- 

 wards becoming yellowish brown. The corners of the case are 

 provided with long horny filaments, with which the fish attaches 

 the eggs to the corals or other objects by swimming round and round 

 them as the egg is being pushed out of the body. Thus the eggs 

 are prevented from being covered by the mud, which is one of 

 the many enemies of the embryo. The development of the embryo 

 can be very well observed owing to the transparency of the case. 

 In tank 21 eggs are hung up close to the glass, and in these the 

 embryos and their movements can be examined. In advanced 

 stages the young fish can be seen 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 distinct. 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. These young stages have of recent years be- 



