ORTHONECTIDiE AND DICYEMID2E 



209 



i 



ther hand, Giard in his first communication described the formation of 

 n epibolic gastrula. 



Development of the Female.— The first stages of 

 cleavage are not known. According to Julin, there is pro- 

 duced here also an epibolic gastrula, the entoderm of which 

 consists even at an earlj stage of a large number of cells. 

 A peripheral layer is said to be differentiated from it into a 

 -layer of cylindrical cells, which, situated under the ectoderm, 

 surrounds the central mass of polyhedral cells. When the 

 embryo has elongated and acquired its coat of cilia, it pre- 

 sents a great resemblance to the embr^^os of the Distomidce 

 and Bothriocephalidce. The outermost of its three cell-layer's 

 would then correspond to the enveloping membrane (HuU- 

 wembran). Out of the second cell-layer, which later be- 

 comes flattened, there is said by Julin to arise a system of 

 extremely delicate muscle fibres, which are found under the 

 ctoderm in the adult female. 



According to Giard and Metschnikoff, during the development of the 

 ffemale a regular blastula makes its appearance, out of which the two 

 ; germ-layers are formed possibly as the result of delamination. 



The above presentation of the life-history and development of the 



Orthonectidae does not rest wholly upon reliable observations, but many 



[gaps in it have been filled by the speculations of the authors. We have 



[adhered chiefly to the account of Julin, for his work is the most complete 



md is an advance upon that of Giard and Metschnikoff. 



M. DICYEMID>E. 



Systematic : van Beneden distinguishes four genera : 



Hcyema, Dlcyemella, Dicyemina, and Dicyemopsis, which are 



listributed among four genera of Cephalopods : Octopus, 



Uedone, Sepia, and Sepiola. They are found in the append- 



Lges of the branchial veins. Whitman admits only two 



genera : Dicyema (with eight cells in the head region) and 



Hcyemennea (with nine cells in the head region). 



The body of the Dicyemidce is elongated. It consists of 



tn outer layer of ciliated cells and a single large axial cell, 



the latter surrounded by the former (Fig. 99 D). At the 



interior end the outer cell-layer is differentiated into a kind 



►f cap [polar calotte]. Elsewhere the outer cells are nearly 



ilike. 



K. H. E. 



