JO LILLIE. [Vol. X. 



the young animal. As parasitism gradually became the fixed 

 habit of the species, the adaptation to the requisite conditions 

 became more and more perfect, until the parasitism became a 

 necessary consequence of the structure, and an indispensable 

 condition of development. 



The parasitism of the glochidium is but one way of securing 

 protection against the injurious effects of fresh water on delicate 

 larvae. The same protection is assured other forms (turbellari- 

 ans, Cyclas, gasteropods) by a foetal development, which takes 

 place either in the body of the mother or in impervious cap- 

 sules. The mode of securing this end among the Unionidae is 

 evidently correlated with the enormous number of young pro- 

 duced. Not that the enormous number of the young made 

 actual viviparous reproduction such, e.g., as in the Cycladidae, 

 impossible, and hence forced, so to speak, the species to devise 

 another means of protection. It is more logical to hold either 

 that the parasitic habit preceded the production of such a mul- 

 titude of larvae, or else that both evolved pari passu,. We 

 cannot, at any rate, suppose that a species could be thus 

 perpetuated if only a few young were produced, so precarious 

 and uncertain is the attainment of the necessary conditions of 

 higher development. 



If we suppose the Unionidae to have been derived ultimately 

 from a marine form, we are offered a hint as to the possible 

 mode of evolution by the present condition of Dreissena, which 

 is evidently undergoing a similar change of habitat. The 

 development of this form is a metamorphosis with a free- 

 swimming larva, which hardly differs from the marine larvae of 

 the same class. It is easy to picture two or three possible 

 courses of evolution open if it becomes completely adapted to 

 life in fresh water (in which case, as all our experience tells 

 us, the larva would be lost). In the one case it might become 

 purely viviparous like Cyclas, and produce but few young ; or 

 the ova might be deposited in impervious capsules (as, e.g., 

 pulmonates) ; or, again, the larva might in some way become 

 adapted to parasitism, with consequent protection to its delicate 

 structure. It is practically certain that in this last event the 

 larva would change in two ways : First, so as to make it an 



