364 CHANGES IN LARVAE. 



this tendency it comes about that when an organism is hatched 

 as a larva many of the organs are still in an undifferentiated 

 state, although the ancestral form which this larva represents 

 had all its organs fully differentiated. In order, however, that 

 the larva may be enabled to exist as an independent organism, 

 certain sets of organs, e.g. the muscular, nervous, and digestive 

 systems, have to be histologically differentiated. If the period 

 of fcetal life is shortened, an earlier differentiation of certain 

 organs is a necessary consequence ; and in almost all cases the 

 existence of a larval stage causes a displacement in order of 

 development of organs, the complete differentiation of many 

 organs being retarded relatively to the muscular, nervous, and 

 digestive systems. 



The possible changes under the second head appear to be 

 unlimited. There is, so far as I see, no possible reason why an 

 indefinite number of organs should not be developed in larvae to 

 protect them from their enemies, and to enable them to com- 

 pete with larvas of other species, and so on. The only limit to 

 such development appears to be the shortness of larval life, 

 which is not likely to be prolonged, since, ceteris paribus, the 

 more quickly maturity is reached the better it is for the species. 



A very superficial examination of marine larvae shews that 

 there are certain peculiarities common to most of them, and it is 

 important to determine how far such peculiarities are to be 

 regarded as adaptive. Almost all marine larvae are provided 

 with well-developed organs of locomotion, and transparent 

 bodies. These two features are precisely those which it is most 

 essential for such larvas to have. Organs of locomotion are 

 important, in order that larvae may be scattered as widely as 

 possible, and so disseminate the species ; and transparency is 

 very important in rendering larvae invisible, and so less liable to 

 be preyed upon by their numerous enemies 1 . 



These considerations, coupled with the fact that almost all 

 free-swimming animals, which have not other special means of 

 protection, are transparent, seem to shew that the transparency 



1 The phosphorescence of many larvae is very peculiar. I should have anticipated 

 that phosphorescence would have rendered them much more liable to be captured by 

 the forms which feed upon them; and it is difficult to see of what advantage it can be 

 to them. 



