THEORY OP THE EMBRYONIC PHASE OF ONTOGENY. 329 



to air breathing. Their " tracheal " gills, instead of like all 

 other gills bringing the blood into close proximity to the water, 

 bring their blood first into contact with air contained in a 

 systena of closed tubes, and then this air into contact with the 

 water. 



In the case of ordinary larvae, the probability of modifica- 

 tions due to adaptation to the environment cannot be denied. 

 If, however, Sedgwick's hypothesis is correct that '' larval 

 history is constructed out of ancestral stages, '' or, in other 

 words, that the larva retains ancestral characters because it 

 retains the ancestral mode of life, then the environment has 

 remained to a large extent constant (at any rate in the com- 

 monest case, that of pelagic larvae), and the changes they are 

 likely to have undergone, instead of being, as Balfour supposed, 

 unlimited, will be comparatively few in number. 



Of these changes reduction in size is the most important. 

 The passage to the adult state is often accompanied by the loss 

 of larval organs, and great changes in those which are retained, 

 necessitating in some cases the complete destruction of their 

 constituent cells, and their reconstruction from rudiments 

 which have retained the embryonic condition (histolysis). It 

 is, therefore, clearly to the advantage of the larva to grow no 

 larger than necessary before it undergoes metamorphosis. 

 Correlated with this loss of size is the frequent disappearance 

 of all traces of segmentation, since this is probably to be 

 regarded as essentially the same phenomenon as vegetative 

 reproduction, only held in check by the individuality of the 

 whole. Metameric series of organs are represented only by 

 those members which are absolutely necessary. Another 

 change which larvae are prone to undergo, is the acquisition of 

 transparency. What results this carries in its train will be men- 

 tioned below. Finally, the occurrence of long spines is a wide- 

 spread phenomenon, though what their precise use is it would 

 be rash to surmise. Possibly they are of a protective nature. 



Let us .now apply these principles in a concrete case, for 

 example the larvse of the Crustacea. 



The characteristic larva of the Entomostraca is the well- 



