DEVELOPMENT. 



less. Moreover, competition is not too severe to prevent some 

 accumulation of food by the parent on behalf of the family. 



On land the conditions are still less favourable to larval 

 transformation. Very early migration is altogether impossible. 

 Any kind of locomotion by land implies muscles of complicated 

 arrangement, and, as a rule, there must be some sort of skeleton 

 to support the weight of the body. The larva, if turned out in 

 a Gastraea condition would simply perish without a struggle.* 

 Nor is great precocity needful. The terrestrial animal is com- 

 monly of complicated structure, active, and well furnished with 

 means of information. It can lay-by for its offspring, and 

 nourish them within its own body, or at least by food stored up 

 in the egg. 



The influence of habitat upon development may be recapitu- 

 lated as follows : 



MARINE HABITAT. Eggs many. Yolk small. Segmentation 

 often regular. Young hatched early. Development with 

 metamorphosis. [The most conspicuous exceptions are Cephalo- 

 poda and marine Yertebrata.] 



FLUVIATILE HABITAT. Eggs fewer. Yolk larger. Seg- 

 mentation often unequal. Young hatched later. Development 

 direct, or with late metamorphosis only. [The most obvious ex- 

 ceptions are Frogs and Toads, which developwith metamorphosis.] 



TERRESTRIAL HABITAT. Eggs few. Yolk large [except 

 where the young are supplied by maternal blood]. Segmenta- 

 tion often partial. Young hatched late. Development without 

 metamorphosis. [An exception is found in Insects, which 

 usually exhibit conspicuous metamorphosis, though the yolk is 

 large, and the type of segmentation partial or unequal.] 



Let us now take up the exceptions, and see whether these are 

 capable of satisfactory explanation. 



1. Cephalopoda and marine Yertebrates, unlike other in- 

 habitants of the sea, develop without metamorphosis. But 

 these are large animals of relatively high intelligence, well able 

 to feed and protect their young until development is completely 

 accomplished. 



* The minute and early larvae of Tcenia and Distomum may appear to contradict 

 this statement. They really inhabit the film of water which spreads over wet grass, 

 though they are capable of enduring dry conditions for a short time, like Rotifers and 

 many Infusoria. 



