362 Mr. J. W. Fewkes on a new 



Fig. 3. Larva of Apaustus ayraulia. 3 a. Ditto, with covering formed of 



leaves. 3 b. Pupa. 

 Fig. 4. Larva of Hypsa nesophora. 4 a. Pupa. 

 Fig. 5. Larva of Entemia camincsa. 5 a. Pupa. 

 Fig. 6. Larva of Philohota bimaculana. 6 a. Ditto, with covering formed 



of leaves. 6 h. Pupa. 

 Fig. 7. Larva of Gonionota pyrobola. 7 a. Pupa. 



XLVIII. — On a new Mode of Life among Medusae. 

 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 structure is so masked by their parasitic 

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

 impossible to recognize their zoological affinities, and it was 

 only when the immature 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. 



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 structure which occur, and it is 

 impossible to give this mode of life a primary importance in 

 theories of origin of species as has been attempted, it is no 

 doubt true that many variations in structure have been 

 derived either directly or by heredity from parasitic an- 

 cestors. 



Nowhere among lower animals is there more likelihood 



* From the ' Proceedings of the Boston Society of Natural History,' 

 vol. xxiii. Communicated by the Author. 



