Mode of Life among Medusre. 363 



that we should find parasitic conditions than among the Me- 

 diisfe. Reflect 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 Jiabitat, a parasitic attach- 

 ment 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 

 suflfered a modification in structure by their attachment is 

 very small. 



Every one who has taken a hand in the most fascinating 

 part of the study of marine zoology, viz. dredging in the 

 ocean, knows how often ascidians, brachiopods, large mol- 

 lusks, 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 iVa<«ca-sliell inhabited by a 

 hermit-crab is not unlike Hydractinia from the underside 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 Medusaj certain recorded 

 cases of parasitism where there is a vital connexion, so to 

 speak, where there is a parasitism or even commensalism of 

 such an intimate character that not only the structure 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 morpho- 

 logical importance, for it affords in some instances at least a 

 means of estimating the modifications 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 regard 

 selection as an all-important factor in the modification of 

 animal structure. 



It is not my purpose, however, to enter into a discussion 

 of this subject, upon which so much has already been said by 

 abler naturalists than myself. I have simply introduced it 

 in preparation for the consideration of new observations 

 bearing upon the question among the jellyfishes. Let me, as 

 an introduction, mention a few instances of modification of 

 Medusan genera by the mode of life called parasitism. 



One of the best known instances of parasitism among 

 Medusae is that of Cunina, which lives parasitic in the stomach 



