LARVAL FORMS. 299 



In spite of the liability of larvae to acquire secondary characters, 

 there is a powerful counter-balancing influence tending towards the 

 preservation of ancestral characters, in that larvae are necessarily 

 compelled at all stages of their growth to retain in a functional state 

 such systems of organs, at any rate, as are essential for a free and 

 independent existence. It thus comes about that, in spite of the 

 many causes tending to produce secondary changes in larvae, there 

 is always a better chance of larvae repeating, in an unabbreviated 

 form, their ancestral history, than is the case with embryos, which 

 undergo their development within the egg. 



It may be further noted as a fact which favours the relative 

 retention by larvae of ancestral characters, that a secondary larval 

 stage is less likely to be repeated in development than an ancestral 

 stage, because there is always a strong tendency for the former, 

 which is a secondarily intercalated link in the chain of development, 

 to drop out by the occurrence of a reversion to the original type of 

 development. 



The relative chances of the ancestral history being preserved in 

 the foetus or the larva may be summed up in the following way : — 

 There is a greater chance of the ancestral history being lost in forms 

 which develop in the egg ; and of its being masked in those which 

 are hatched as larvae. 



The evidence from existing forms undoubtedly confirms the 

 a priori considerations just urged \ This is well shewn by a study 

 of the development of Echinodermata, Nemertea, Mollusca, Crustacea, 

 and Tunicata. The free larvae of the four first groups are more 

 similar amongst themselves than the embryos which develop directly, 

 and since this similarity cannot be supposed to be due to the larvae 

 having been modified by living under precisely similar conditions, 

 it must be due to their retaining common ancestral characters. In 

 the case of the Tunicata the free larvae retain much more completely 

 than the embryos certain characters such as the notochord, the 

 cerebrospinal canal, etc., which are known to be ancestral. 



Types of Larvae. — Although there is no reason to suppose that 

 all larval forms are ancestral, yet it seems reasonable to anticipate 

 that a certain number of the known types of larvae would retain the 

 characters of the ancestors of the more important phyla of the animal 

 kingdom. 



Before examining in detail the claims of various larvae to such a 

 character, it is necessary to consider somewhat more at length the 

 kind of variations which are most likely to occur in larval forms. 



1 It has long been known that land and freshwater forms develop without a 

 metamorphosis mnch more frequently than marine forms. This is probably to be 

 explained by the fact that there is not the same possibility of a land or freshwater 

 species extending itself over a wide area by the agency of free larva?, and there is, 

 therefore, much less advantage in the existence of such larvae ; while the fact of such 

 larvffi being more liable to be preyed upon than eggs, which are either concealed, or 

 carried about by the parent, might render a larval stage absolutely disadvantageous. 



