ORTHONECTID« AND DICYBMIDiE 209 



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

 an epibohc 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 polyhedral cells. When the 

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

 sents a great resemblance to the embryos of the Distomidce 

 and BothriocephalidcB. The outermost of its three cell-layers 

 would then correspond to the enveloping membrane (Hilll- 

 memhran). Out of the second cell-layer, Avhich later be- 

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

 extremely delicate muscle fibi^es, which are found under the 

 ectoderm in the adult female. 



According to Giaed and Mbtschnikoff, 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 develojimenA of the 

 Orthoneetida3 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 

 and is an advance ujjon that of Giakd and Mbtschnikoff. 



II. DICYEMID/E. 



SrsTEMATiC : VAN Beneden distinguishes four genera : 

 Dicyema, Dlcyeinella, Dicyemina, and Dicyemopsis, which are 

 distributed among four genera of Cephalopods : Octopu^^, 

 JEledone, 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 Dicyemidm 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 



