Fewkes.] 390 I Nov. 2, 



of those which have suffered a modification in structure by their 

 attachment is very small. 



Everyone 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 mollusks and other ani- 

 mals 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 modi lied by 

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

 inhabited by a hermit crab, is not unlike Hydractina from the un- 

 derside 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 Medusae, certain recorded cases 

 of parasitism 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 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 morphological importance, for it af- 

 fords, in some instances, at least a means of estimating the modi- 

 fications of structure which may result in Medusae from parasitic 

 habits. They introduce into the discussion of the theory of evo- 

 tion a series of facts which may well be carefully considered by 

 those who ascribe to selection an all-important factor in the mod- 

 ification 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 prepara- 

 tion for the consideration of new observations bearing upon the 

 question among the jelly-fishes. Let me, as an introduction, men- 

 tion 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 of another 

 Medusa, Geryonia. We, undoubtedly, have, in this case, a mod- 

 ification of the parasite by its peculiar mode of life in the host, 

 although a reciprocal effect on the host is not recognizable. 



Less known than Cunina, although quite as interesting, is that of 



