1887.] 389 TFewkes. 



A NEW MODE OF LIFE AMONG MEDUSAS. 



BY J. WALTER FEWKES. 



Several pamphlets and one or two books have been written on 

 the influence of parasitism in the modification of animal structure. 

 Perhaps nowhere do we find this mode of life better illustrated 

 than among certain of the Crustacea, where the anatomical struct- 

 ure is so masked by their parasitic habits that for a long time in 

 the history of research it was impossible to recognize their zoo- 

 logical affinities, and it was only when the immature stages in the 

 growth were studied and larval conditions, unaffected by parasit- 

 ism, had been investigated, that the true relationships of the group 

 could be discovered. 



What we find in the so-called Lernean worms exists wherever 

 parasitism is found among animals. It may, in fact, be concluded 

 that ordinarily in parasites there is a degradation in structure, or 

 at all events such a modification as to lead to important changes 

 in anatomy and external form. 



It would seem that among the lowest animals we ought to find a 

 larger number of parasitic genera than among the higher. While 

 there is little doubt that there is more variety in lower animals, I 

 am not so confident that this mode of life has led to as great 

 modifications in structure here as might be expected. While we 

 cannot ascribe to parasitism the many variations in animal struct- 

 ure which occur, and it is impossible to give this mode of life a 

 primary importance in theories of origin of species as has been at- 

 tempted, it is no doubt true that many variations in structure have 

 been derived either directly or by heredity from parasitic ances- 

 tors. 



Nowhere among lower animals is there more likelihood that we 

 should find parasitic conditions than among the Medusae. Re- 

 flect for a moment that the young of a majority of these animals 

 live attached to submarine objects, and it seems easy to see how 

 by changing its habitat a parasitic attachment to another animal 

 might easily take place. Considering the probabilities, however, 

 although the number of genera which might be mentioned as liv- 

 ing upon other animals is large, the number of recorded instances 



