1S88.] MICKOSCOPICAL JOUENAL. 207 



A. New Mode of Life Among Medusae.* 



By J. WALTER FEWKES, 



CAMBRIDGE, MASS 



Much has been written on the influence of parasitism in the modification 

 of animal structure. Perhaps nowhere do we find this better illustrated than 

 among certain of the Crustacea, where the anatomical structure is so masked 

 by parasitic habits that for a long time in the history of research it was im- 

 possible to recognize their zoological afiinities, and it was only when the im- 

 mature stages in the growth were studied and larval conditions, unaffected 

 by parasitism, had been investigated, that the true relationships of the group 

 could be discovered. 



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. 



Nowhere among lower animals is there more likelihood that we should find 

 parasitic conditions than among the Medusfe. The young of a majoritv 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 living upon other animals is large, 

 the number of recorded instances of those which have suffered a modification 

 in structure by their attachment is very small. 



Everyone who has taken a hand in dredging in the ocean knows how often 

 ascidians, brachiopods, large mollusks, and other animals are brought up with 

 attached hydroids growing upon them. These hydroids, in one sense, are not 

 parasitic, as they draw no nourishment from their hosts, nor are they at all 

 modified by their mode of life. For instance, Hydractinia, from a Natica 

 shell inhabited by a hermit ci^ab, is not unlike Hydractina from the under- 

 side of a floating bell-buoy. Obelia from the stalk of Boltenia is specifically 

 the same as Obelia on a submerged log. In these and similar instances, for 

 they are numerous and varied in nature, there is no resultant modification 

 either of host or parasite, as the attachment is in no way vital or intimate. 



There are, however, among the Medusce, certain recorded cases of para- 

 sitism where there is a vital connection so to speak, where there is a parasitism, 

 or even commensalism, of such an intimate character that not only the struct- 

 ure of the parasite, but also even that of the host itself is modified. It is a 

 study of these cases which has a most interesting morphological importance, 

 for it affords, in some instances, at least a means of estimating the modifica- 

 tions of structure which may result in Medusae from parasitic habits. They 

 introduce into the discussion of the theory of evolution a series of facts which 

 may well be carefully considered by those who ascribe to selection an all-im- 

 portant factor in the modification of animal structure. 



One of the best known instances of parasitism among Medusae is that of 

 Cunina which lives parasitic in the stomach of another Medusa, Geryonia. 

 We, undoubtedly, have, in this case, a modification of the parasite by its pe- 

 culiar mode of life in the host, although a reciprocal effect on the host is not 

 recognizable. 



A most interesting instance of parasitism, and consequent modification 

 among Medusce, is found in the problematical organism, Polypodium. This 

 undoubted hydroid is found parasitic in the ova of the sturgeon while in the 

 body of the fish. We have in Polypodium, as described by Ussow, a hy- 



* From Proceedings of Boston Society of Natural History. 



