PLATHELMINTHES 169 



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

 dinm in form. 



Quite different from the larval forms hitherto considered is a planarian 

 larva found by A. Agassiz, which he 

 ascribes to Planaria angidata. 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- F''^' ^l- -Larva of Planaria an- 



, /,,,.,, „ ,, , -, giilata (?) (after A. Agassiz, from 



lopment that it becomes flattened and balfouk's Compayative Emhryo- 

 takes on the form of a turbellarian. logy). 

 Unfortunately a confirmation of Agas- 

 siz's short communication has not yet appeared. 



II. Tricladida. 



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 polychroa is such that thei*e 

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

 Dendroccelum 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 amoebae 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," Proceedings of the Society of N aturalists at Kasan, 1872- 

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



