560 THE ANATOMY OF INVERTEBRATED ANIMALS. 



they meet, they surround an opening through which one end 

 of the axial cell protrudes. This corresponds with the oral 

 pole. 



Before the young Dieyema thus developed leaves the 

 body, which it generally does by traversing the oral pole 

 (though it may make its way out through the parietes), two 

 embryos of the same kind appear within its axial cell. 



Thus the nematogenous Dieyema gives rise by agamo- 

 genetic process to new Dicyemas. 



In the Rhombogena the germs are developed in from two 

 to five special nucleated parent cells, the origin of which is 

 not known. They are found imbedded in the protoplasm of 

 the axial cell, and the germs are developed endogenously 

 from the protoplasm of the parent cell, the nucleus of which 

 remains unchanged. The germs undergo division, and be- 

 come spheroidal bodies composed of two kinds of cells, small 

 and large. Each of these bodies is converted into an infu- 

 soriform, bilaterally symmetrical embryo, which consists of 

 an urn, a ciliated body, and two refractive bodies. 



The urn, situated on the ventral side of the embryo, is 

 composed of a capsule, a lid, and contents. 



The latter are four granular masses, each of which con- 

 tains many nuclei, and eventually becomes covered with cilia. 

 The refractive bodies take their origin in two adjacent cells. 

 They partially cover the urn in front, arid form the largest 

 portion of the dorsal face of the embryo. The ciliated body 

 consists of ciliated cells, and forms the caudal portion of the 

 embryo. 



While the vermiform embryo becomes a Dieyema in the 

 body of the Cephalopod on which its parent is parasitic, the 

 infusoriform embryo is set free, and probably serves as the 

 means by which the parasite is transmitted from one Cepha- 

 lopod to another. 



Professor E. van Beneden compares the cortical layer of a 

 Dieyema to the ectoderm, and the axial cell to the endoderm 

 of a Metazoon ; and the mode of production of the embryo 

 to the process of epiboly in the Metazoa. But, from the 

 complete absence of any mesoblastic layer, he proposes to 

 establish a new division of Mesozoa, intermediate between 

 the Protozoa and the Metazoa, for the Dicyemida. 



