THE DEVELOPMENT OF ORGANISMS 



789 



of a larva. It is plain, therefore, that the occurrence of larvae is no 

 necessity, though undoubtedly very common. We see larvae in the 

 life-history of frogs, many fishes, lampreys, most sea-squirts, many 

 molluscs, all the higher insects, most crustaceans, not to speak of 

 worms, starfishes, and other animals of relatively low degree. 



As long as development is proceeding within an egg-shell or 

 egg-envelope, we speak of an embryo. It is in no way able to fend 

 for itself. But when this embryo is hatched out by the breaking of 

 the egg-shell or the egg-envelope we call it a young creature if it is 



•"^3— 





•'^yOy^y^y>>ff^y^.<^^- 



Fig. 134. 



Part of the Life-History of the Common Eel {Anguilla vulgaris). After 

 Schmidt. I, a very young larva; II-III, stages of Leptocephalus or 

 Glass-Eel growth; IV, the beginning of the change into the cylindrical 

 elver form; V, an elver, ready to ascend the river, about three years. old. 



in a general way a miniature of its parent; but we call it a larva if 

 it shows a more or less different plan and aspect. Thus no one could 

 say that a caterpillar is at all like a moth or a butterfly, and it takes 

 a strong imagination to see the frog in the tadpole. 



The larval eel in the open sea is so unlike an eel that it was called 

 by a different name — Leptocephalus — for many years before it was 

 known to be what it is. Similarly the fresh- water young stages of 

 the big marine lamprey were for many a year called Ammocoetes 

 (or "niners") before their parentage was proved. A hatched-out 

 independent young creature is called a larva when it has a distinctive 

 organisation of its own, different from that of the adult animal. 



To put it in another way, the larva has to undergo some sort 



