146 PISCES. 



and is ejected througli an orifice at the hind margin 

 of the gill-cover. 



In many Fishes there is a large bladder situated 

 within the body, between the spine and the bowels. 

 It assumes various forms, and is always filled with 

 air, which, in Marine Fishes, is principally com- 

 posed of oxygen. It is supposed to be connected 

 with the buoyancy of the animal, and hence is 

 often called the swimming -Ijladder ; but there are 

 structural reasons for considering it to be the first 

 rudimentary form of an air-breathing lung. 



Fishes, like all other Vertebrata, have the 

 sexes separate. Among the Sharks and Skates, the 

 male may be known by a long cylindrical appen- 

 dage at the inner edge of each ventral fin, which is 

 entirely wanting in females. In general, however, 

 the females are distinguishable from the males only 

 by the body being deeper and fuller, while the males 

 have the head and gills more developed. 



With a few exceptions, more apparent than real, 

 the young are produced from eggs^ which, being 

 deposited at regular periods, and usually in vast 

 numbers, are commonly called spawn. The mass 

 of immature eggs in the body of the female fish is 

 well known under the name of hard roe. The roe 

 of a Cod has been computed to contain six millions 

 of eggs. 



For the most part, the spawn is committed to 

 the* waters with but little precaution for its pre- 

 servation : sometimes it is twined around a sea- 

 weed, sometimes laid like beads within the valves 

 of a dead shell, sometimes dropped into a furrow, 

 ploughed in the bottom-gravel. But a few ex- 

 amples have recently come under the observation 

 of naturalists, in which an elaborate nest is con- 



