PLATHELMINTHES 



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

 dium in form. 



Quite different from the larval forms hitherto considered is a planarian 

 larva found by A. Agassiz, which he 

 ascribes to Planaria 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 ; 

 it is only in the course of further deve- 

 lopment that it becomes flattened and 

 takes on the form of a turbellarian. 

 Unfortunately a confirmation of Agas- 

 siz's short communication has not yet appeared. 



Fig. 81.— Larva of Flanaria an- 

 gulata (?) (after A. Agassiz, from 

 Balfour's Com'paYative Embryo- 

 logy). 



I 



II. Teicladida. 



The difference between the development of the fresh- 

 water Bendroccelida (Triclads) and that of the Polycladida is 

 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, there 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 joolychroa is such that there 

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

 De7idroccelum lacteum, 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 radial 

 arrangement, and fill the remaining space of the cocoon. 

 They are able to move like amcebse 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 SocieUj of Naturalists, vol. v. (Odessa), 1887, — was unfortunately 

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

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

 73 (see Leuckakt, Jahresber. Arch. f. Naturg., Jahrg. xl., Bd. ii., 1874). 



