No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 69 



But why these parts are so small is a different question. 

 We can recognize in this the degrading influence of parasitism. 

 It would, indeed, be strange if this mode of life, which can so 

 profoundly influence animals which have become adapted to it, 

 as to render it a matter of speculation in what corner of the 

 animal kingdom to classify them, had not left a deep imprint 

 on the organization of this larva. The effect of parasitism is 

 to exaggerate all organs essential, and to eliminate all that are 

 inessential, to the parasite. This is precisely what has taken 

 place in the glochidium. The larval thread, the strong muscle, 

 the heavy shell with its hooks, and the larval mantle, are all 

 essential to its peculiar mode of parasitism. The foot, the 

 mouth, the intestine, the heart, etc., etc., are all inessential. 

 The former have thus been enormously exaggerated, becoming 

 precociously impressed on the cleavage of the ovum ; the latter 

 have been reduced to mere rudiments, the reduction also leav- 

 ing its imprint on the segmentation ; but they have not been 

 eliminated, because they are functional organs in the adult. 



This is one of the most interesting of all cases of parasitism, 

 because we have an animal fully equipped as a larva for para- 

 sitic existence, and later leading an independent life. It shows 

 us how far parasitism can go without eliminating the possibili- 

 ties of a higher evolution. It seems strange that the parasitism 

 should finish with the larval life ; but that it does so, and that, 

 despite its short duration, the preparation therefor should so 

 profoundly alter the characters of the larva, is one of the best 

 examples of the oft-emphasized fact that natural selection deals 

 no less with the larva than with the adult. We might search 

 the animal kingdom through without finding a better example. 

 What a bountiful supply of transitory organs of offense and 

 defense has nature supplied to this larva ! and all that a pass- 

 ing and purely larval condition should be ensured in the greatest 

 possible number of cases ! 



Another important question which suggests itself, is : Why 

 has this brief parasitic period been intercalated in the life 

 history of this animal .? I think that Schierholz answered the 

 question very satisfactorily when he said that it was to avoid 

 the injurious action of the fresh water on the delicate shell of 



