A Contribut. to the Embryol., Life-history, aud Classificat. of the Dicyemids. 33 



seasonal influences. Althoiigh, so far as I can judge, the cases in whicli 

 it can be said with a veiy high degree of pvobability that only Rhomb- 

 ogens or only Nematogens are present are comparatively few, stili 

 we cannot deny their importance. They cau not of coiirse be accepted 

 as proof that one form passes into the other , but they are more easily 

 reconciled with such a transition than with any contradictory hypothesis 

 that has yet been oifered. My observations on these points bave been 

 made chiefly on the parasites of Eledone moschata , and bere I find only 

 one case recorded in which Rhombogens alone were found , and eleven 

 in which Nematogens were similarly isolated. Many cases were noticed 

 in which both kinds were about equally numerous , but a much larger 

 number of cases in which there was a more or less decided numerical 

 preponderance in favor of one or the other form. 



In the early part of March I happened to receive only small ex- 

 amples of Eledone moschata for several days in succession . In ali I found 

 the Nematogens very abundant, and in some, Rhombogens appeared to 

 be whollyabsent. The possibility of a seasonal occurrence of the Rhomb- 

 ogens, as suggested by Van Beneden , then presented itself : but this 

 idea was not sustained by subsequent examinations. I soon recognized 

 a remarkable degree of correspondence between the size of the Cephalo- 

 pod and the numerical relations of the two kinds of Dicyemids. Large 

 Eledones invariably inclosed Rhombogens, often in much greater abuu- 

 dance than Nematogens ; while very small Eledones showed obverse re- 

 lations, sometimes furnishiug only Nematogens. I made numerous ex- 

 aminations during March and Aprii with special reference to this point, 

 and a comparison of the results obtained from Eledones weighing from 

 15—50 grams with those from Eledones weighing from one to several 

 hundred grams, show that the correspondence above stated is not an ex- 

 ception but an invariable rule. As examples the following cases are given. 



Apr. 24. E. moschata 2-5 grms. Dicyema moschatmn ; mostly Nematogens, but 



a consideiable nuuiber of Rhombogens among 

 the smaller individuals . 



- - - 24 - Dicyema moschata ; ali Nematogens. 



- 2.5. - 15 - One tuff, or colony of large clear Nematogens 



(D. moschattim) without verruciform cells; 

 a large tract of short Nematogens ofthe same 

 species , mostly with two verruciform cells. 

 In auother part of the renai organ , long Ne- 

 matogens {Dicyemennea Eledones). 

 No. Rhombogens. 



- 24. - 185 - Mostly Rhombogens (J). moschatum). A few 



Nematogens inclosing two or three embryos 



Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. IV. 3 



