Fishes. 89 



slit from side to side, and the neclc has five or more gill-slits on each 

 side, but no gill-cover. The eyes are provided with moveable eye-lids, 

 the nostrils communicate with the month, forming a «hare-lip», a con- 

 dition passed through by the embryos of all higher Vertebrates. These 

 external characteristics alone would distinguish the Sharks from the bony 

 fishes, biit 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 , Seyllium , of which two species exist in 

 European seas , S. eatulus , the Rock Dog-fish , or Bounce , as it is 

 frequently called, and S. eanleula, the Little Dog-fish or Morgay. The 

 first is aboxit 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. 



The female lays its eggs singly on branches of coral (e. g. Tank Nr. 10 

 on the right) or attaches them to bushes or rocks. They are contained 

 in rectangular, leathery capsules known as MermaicVs or Sailor's purses, 

 semi-transparent and white when first laid, but afterwards becoming yel- 

 lowish brown. The corners of the case are provided Avith 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 , wdiich is one of the many enemies of the embryo. The de- 

 velopment of the embryo can be very well observed owing to the trans- 

 parency of the case; this reveals, in advanced stages, the young fish with 

 a bunch of external gills, at either side of the neck, aiad the large yolk 

 attached by a hollow cord through which the food substance is passed 

 into the alimentary tract. Gradually the provisional gill-threads disap- 

 pear, and the colouring of the body becomes more distinct. When the 

 yolk is consumed, the young Dog-fish is ready to hatcii ; it forces its 

 w^ay 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 Nr. 10, and the fishermen often bring in branches 

 of coral and other objects, to Avhich the eggs are fixed. (These may be 

 seen in tank Nr. 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 provides. Economically, hoAvever, 

 the Dog-fish is of very little use. Its flesh is of very indifterent quality 

 and only eaten by the poor; the skin (shagreen) is used for polishing, 

 and the liver yields a certain amount of oil. 



Almost all shai*ks are viviparous , so that Seyllium is an exception 

 to the rule. 



12 



