PLATHELMINTHES 



169 



seems to resemble Goette's larva. It is said also to resemble the pili- 

 ilium in form. 



Quite different from the larval forms hitherto considered is a planarian 

 larva found by A. Agassiz, which he 

 ascribes to Phaiaria angulata. This 

 larva, in which a branched intestine is 

 already present, shows a distinct exter- 

 nal segmentation corresponding to the 

 lateral branches of the intestine (Fig. 



81). At first the body is cylindrical; 



-, ■ 1 • ii ü Ü i-u 1 Fig. 81. —Larva of Plnnaria aii- 



it IS only m the course of further deve- „! , , . , . 



/ gulata (?) (after A. AG.iSSiz, from 



lopment that it becomes flattened and B^^o^^.g Comparative Emhryo- 



lakes on the form of a turbellarian. Ugy). 



Unfortunately a confirmation of Ag.\s- 



siz's short communication has not yet ai)23eared. 



II. Tricladida. 



The difference between the development of the fresh- 

 water Dendroccelida (Trielads) and that of the Polycladida i.s 

 to be explained by the fact that it takes place under alto- 

 gether different conditions. In the cocoons laid by fresh- 

 water Dendrocoeles, which are disproportionately large as 

 compared with the size of the animal, thei^e is found, in 

 addition to the egg-cell, a large number of yolk-cells. 

 According to Metschnikoff (No. 15), the proportion of the 

 two kinds of cells in Planaria polychroa is such that there 

 are four to six egg-cells to about ten thousand yolk-cells. In 

 Dendrocojhim Jacteum, on the other hand, twenty to forty 

 egg-cells are present in one cocoon (Iijima, No. 8 ; Hallez, 

 No. 7). The yolk-cells surround the egg-cells in a I'adial 

 arrangement, and fill the remaining space of the cocoon. 

 They are able to move like amoebte by sending out pseudo- 

 podia. 



As soon as the first stages of cleavage have taken place in 

 the egg-shell (Figs. 82 and 83), this remarkable phenomenon 

 occurs : the blastomeres do not remain united, but move far 



Russian Society of Naturalists, vol. v. (Odessa), 1887, — was unfortunately 

 inaccessible to us, as was also one by Salensky : "The Development of 

 Enterostomum," Froceediuys of the Society of Naturalists at Kasan, 1872- 

 73 (see Leuckakt, Jahreshcr. Arch.f. Natury., Jahrg. xl., Bd. ii., 1874). 



