406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



formation which connect the extreme, scarcely recognizable forms, 

 with the normal ones. Taken all together, parasites more than any 

 other category of organisms, are a collective and striking illustration 

 of adaptation. Nowhere else does structure appear so sharply out- 

 lined as modeled by the kind of life, nor does preadaptation appear 

 less probable. 



The adult is not alone in being thus influenced ; the young stages 

 are no less influenced, and the conditions of entering the host, often 

 the necessar}^ passage through an' intermediary host (and conse- 

 quently the enormous destruction of individuals in course of develop- 

 ment) are in correlation with adaptive peculiarities of the larvae, 

 frequently with phenomena of embryonic multiplication dependent 

 on asexual reproduction. Comparison of the various groups is very 

 suggestive in this connection, because among them we can see par- 

 allel divergences from normal embryological development carried out 

 in perfectly distinct series, as if the action of the environment were 

 effective in directly modeling, in the purely Lamarckian sense, the de- 

 velopment and structure of parasites. 



Let us not suppose, however, that the problem is simple. Condi- 

 tions which appear to be similar sometimes bring about the most 

 opposite results. In this connection the examples of the parasitic 

 copepods is a striking one. They attach themselves to their host 

 after a period of free life during which they have in general gone 

 through the same principal stages of larval development as the free 

 forms, reaching the stages known as cyclopoid; then they attach 

 themselves, deform themselves, and degrade themselves in the 

 usually understood sense. Mr. Mesnil and I have just finished 

 studying the truly astonishing regression of the genus which we 

 have called Xenocoeloma which actually succeeds in literally appro- 

 priating a part of the organs of its host. It is impossible to imagine 

 a more complete degradation and more intense parasitism. As a 

 contrast here is another type of copepods, the Monstrillidae. They 

 enter their host in the nauplius larval stage. They then lose all 

 their appendages and become a simple undifferentiated cellular mass 

 which undergoes its development in the circulatory system of an an- 

 nelid, 3 that is, under conditions of supreme parasitism. We should 

 expect that from this mode of life would result a creature as de- 

 graded as possible. Nothing of the kind. On the contrary from 

 this juvenile parasitism there arises a copepod splendidly endowed for 

 life in a free medium and armed with powerful swimming ap- 

 pendages for pelagic life. One detail of its organism was, however, 

 paradoxical, constituting an enigma before all the initial parasitic 



'One species has been found parasitic in a mollusk (Odostomia). See Pelseneer, Bull. 

 Scien. France-Belgiquo, vol. 47, 1913. 



