LARVAL FISHES : THEIR METAMORPHOSES. 121 



familiar. Marine " worms," star - fishes, sea- 

 urchins, " shell-fish," and crustacea-crabs and 

 lobsters and their kind, furnish us with many 

 most curious and often wonderfid and com- 

 plicated larval forms. 



When we come to reflect upon this matter a 

 little, we remark that larvae are characteristic of 

 those animals which lay small eggs, whilst those 

 which lay large eggs produce young which re- 

 semble the parents in all respects save size and 

 perhaps colour, or other minor details. 



But what has the size of the egg to do with 

 the matter 1 Just this. That which we know 

 as the egg, the hen's egg for example, contains 

 within the familiar shell two very important 

 parts — the germ which is to develop into the 

 chick, and a large store of food material, which 

 we call the yolk. The amount of this yolk in 

 the bird's egg is, relatively to the size of the 

 germ — enormous; quite sufficient, indeed, to 

 suppoit the developing chick for a comparatively 

 long period. By the time this yolk is all absorbed 

 the development of the chick is almost complete, 

 little more than increase in size being now 

 necessary. The eggs of fishes, on the contrary, 

 never contain much yolk, only sufficient to sup- 

 port embryonic development — as opposed to 

 larval development — for a short time. So soon 

 then as this yolk is all absorbed embryonic de- 

 velopment ends, and the larval development 

 begins. The larval development often begins so 

 early that special or temporary feeding and loco- 

 motory apparatus have to be introduced to serve 

 whilst the permanent organs are being built up. 



