THE RHOMBOZOA 



149 



They then have the appearance of small white threads ; but on the death 

 of the host the cells, with the parasite, drop off into the renal sac, where 

 they are to be found swimming freely, but they never enter the various 

 other cavities with which this renal sac communicates. The Hetero- 

 cyemids do not appear to be thus fixed. 



Fio. I. 



Heterocyemida (after van Beneden). A, Nematogen 

 of Conocyema polymorpha (van Ben.); a, the anterior 

 " verruciform cells" representing (?) the "polar cap" 

 of Dicyemida ; ft, their refringent contents; i, "cili- 

 ated " processes (N.B. The "cilia" are motionless and 

 retractile) ; b, ectoderm cells of body ; c, nucleus of 

 axial cell ; d, protoplasm of axial cell ; g, germs, in 

 outline, the largest being a fully formed vermiform em- 

 bryo. B, Nematogen of Microcyema vespa ; a, anterior 

 cells with refringent contents ; 6, ectoderm of body ; 

 d, axial cell ; c, its nucleus ; g, germs. (7, " Wagener's 

 embryo" of Microcyema; a, cell (or cells) containing 

 refringent bodies ; b body ectoderm (of which there 

 are only four cells in all) ; c, nucleus of axial cell (d). 

 (The embryo of Conocyema resembles this in general 

 structure, but is without refringent bodies ; the cell 

 (or cells) containing them being represented by four 

 cells ; there are also more ectoderm cells.) 







0. 



Pro. II. 



Dicyema typus, E. v. Ben. (after 

 Whitman). Surface view of a 

 young nematogen, from the left 

 side ; although cilia are indicated 

 only round the edges, it is to be 

 understood that the whole surface 

 is ciliated, a, polar cells in two 

 circlets of four ; c, parapolar cells, 

 followed by two dorsal and one ven- 

 tral cell ; t, terminal cells, which 

 in some species may contain refrin- 

 gent bodies, which are here con- 

 fined to the verruciform cells (v). 



Leaving aside these aberrant and rare Heterocyemida, it is only 

 necessary to describe the structure of the Dicyemida. 



The elongated cylindrical body is pointed at one end, which is 

 regarded as the posterior, while the anterior end is thicker, owing to the 

 specialisation of certain of the ectoderm cells to form the " polar cap " 

 (" calotte " of Whitman). The ectoderm is everywhere one cell thick, the 



