134 PROFESSOR E. VAN BENEDEN. 



of cubical cervical cells belonging to the ectoderm, which 

 are filled with finely granular protoplasm i their cilia are 

 large and short, and they are regularly placed round the 

 oral pole, whence they are known as polar cells ; they are 

 always ranged in two rows of four cells each, which together 

 constitute the polar velum (fig. 2). The heads of Dicyema 

 Clausiana, Dicyemella Wdgeneri, and Dicyemella 3Iulleri, are 

 bilaterally symmetrical, and are composed wholly of polar 

 cells. The head of Dicyemi?ia Kollikeriana consists of four 

 conical cells placed in a first row, and of five prism-like 

 cells placed between them as a second row, the whole nine 

 of these polar cells forming a very granular opaque body, 

 ivhich veils the anterior extremity of the endodermic cell. 

 There is a second kind of cell found in the Dicyemidse, to 

 which the name of " parapolar cells " has been given (fig. 2, 

 p c), because they are always adherent to the polar cells, and 

 frequently resist even the dissociating action of acetic acid. 

 These cells have a convex internal face, and are nearly 

 elliptical when seen in optical section ; they contain finely- 

 granular protoplasm, but never any of those refracting 

 globules which are constantly met with in the cells which 

 make up the substance of the ectoderm. In Dicyemina 

 Kollikeriana there are two of these cells, and in Dicycmopsis 

 macrocephalus four, two ventral and two dorsal ; they are 

 in this instance very large, and the external face shows a more 

 Or less deep depression, which tends to divide the cell into 

 two parts (see fig. 2). Glancing now at the ectodermic cells 

 which compose the trunk, we find their internal or deep faces 

 are applied to the surface of the endodermic cell, whilst the 

 external, which are always convex, are covered with long 

 vibratile cilia. The number of these cells is probably con- 

 stant in the same species. In Dicyema typus the whole 

 number of cells composing the body is twenty-six in the 

 adult, for there is one endodermic cell, eight polar, two 

 parapolar, and fifteen ordinary ectodermic. Similarly in 

 Dicyemina there are twenty-six cells, of which one is endo- 

 dermic, nine are polar, two are parapolar, and fourteen are 

 ectodermic. This is readily ascertainable, for the body is 

 broken up under the eyes of the observer by the action of 

 dissociating agents, whilst the parts still retain their relative 

 positions. The form of the cells in embryos and young 

 specimens is often cubical, but with age they elongate in the 

 direction of the main axis of the body, and eventually be- 

 come fusiform. In well-developed individuals these cells 

 attain a gigantic size. The last two cells together bound 

 a cvlindrical cavitv, terminated in a cul-fl'^-^n'- ^^^/^ -lU-rrU 



