On the Life-History and Development of the Genus Myzostoma. 563 



M. cirriferum. Metschnikoff's paper ^ on the development of the 

 group referred chiefly to M. cirriferum. I have not given in detail the 

 development of this species , firstly because the segmentation has al- 

 ready been described and figured by Metschnikoff , and secondly, 

 because the development dififers only in the minutest details from that 

 of 31. glahrum. A few words may be added here. The egg does not 

 differ appreciably in size from that of M. glahrum. The eggs of M. 

 cirriferum however seemed to be a little more transparent than those of 

 M. glahrum. The segmentation is practically identical in both species. 

 But when the first two Segments are formed the larger of the two does 

 not exceed the smaller in size to the same degree as in M. glahrum. This 

 is even more the case w^hen three Segments are present , then all three 

 do not diflfer mnch in size. Still the future hypoblast cell is larger 

 than the other two. This has been correctly figured by Metschnikoff. 

 It is due perhaps to the fact , that the q^^ of M. cirriferum possesses 

 rather less yolk than that of M. glahrum. Further it may be remarked 

 that in Metschnikoff's fig. 6 , pl. XIII one of the segments has ob- 

 viously been overlooked and is not figured. My figure 10 shows this 

 stage. 



In the further development the larvae are exactly alike in both 

 species. In fig. 20 I have giveu a larva of M. cirriferum at the age of 

 three days , and by its side fig. 19 one of M. glahrum aged four days. 

 The resemblances, almost amounting to identity, vnW be obvious at once. 

 It is the same in the further development. 



Systematic position. 



It may be questioned whether any animai has been so pushed 

 about from one division of the animai kingdom to another as Myzostoma. 

 The most diverse views have been' and are still held as to its proper 

 systematic position. 



This is no doubt due in great part to the absence of any complete 

 knowledge of its development. But now that this gap is in great part 

 at any rate filled up , it may be hoped that the genus will be allowed 

 to enjoy a little well-earned repose. 



Its discoverer, F. S. Leuckart, regarded it as a Trematode. Jo- 

 hannes Müller and Lovén were inclined to place it among the Tardi- 

 grada , Lovén at the same time holding it to have affinities with the 

 higher Annelida. Siebold looked upon it as a link between the Chae- 



» 1. c. 



37* 



