ORTHONECTID^ AND DICYEMIDJl 209 



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

 an 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 early 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 polj'hedral cells. When the 

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

 sents a great resemblance to the embrj'os of the Distomidoi 

 and Bothrtocephalidce. The outermost of its three cell-layers 

 would then correspond to the enveloping membrane (Hi'dl- 

 memhran). 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 

 ectoderm in the adult female. 



According to Giard and Metschnikoff, during the development of the 

 female 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 

 Orthonectidffi does not rest wholly upon reliable observations, but many 

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

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

 and is an advance upon that of Giard and Metschnikoff. 



II. DICYEMID>E. 



Systematic : van Beneden distinguishes four genera : 

 Dicyema, Dlcyemella, Dicyemina, and Dicyemopsis, which are 

 distributed among four genera of Cephalopods : Octopus, 

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

 ages of the branchial veins. Whitman admits only two 

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

 Dicyemennea (with nine cells in the head region). 



The body of the Dicyemidre is elongated. It consists of 

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

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

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

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

 alike. 



K. H. E. p 



