150 



THE RHOMBOZOA 



cells are flat, lozenge-shaped, or rhomboidal, and all carry cilia, which, on 

 the " head," are shorter and denser than on the " trunk." The " polar 

 cap " consists of two circlets of cells. The apical circlet always consists of 

 four equal " propolar " cells (Whitman) ; the second circlet consists of 

 either four "metapolar" cells (Dicyema\ or of five cells (Dicyemennea). 1 

 The polar cells are symmetrically arranged in the young when the head 

 is " orthotropous," but by the unequal growth of one, the dorsal, surface 

 the polar cap becomes pushed forwards on this side, so as to lie more or 

 less obliquely ; it is then " plagiotropous " (Fig. III.). The polar cells, when 

 eight, correspond in position, two of each circlet being dorsal and two ven- 

 tral. In Dicyemennea the extra metapolar cell is dorsal (Fig. IV.). The 

 whole organism recalls to some degree a Trematode "miracidium," and these 



cap cells remind one of similarly placed 

 cells in such species as List, tereticolle. 



-CUA 



Fio. III. 



Dicyema macrooephalum, E. van Ben. (altered from 

 Whitman). A portion of a rhombogen seen from the 

 right side ; the plagiotropous cap is represented in 

 optical section, ad, av, dorsal and ventral propolar 

 cells ; bd, bv, dorsal and ventral metapolar cells ; e, 

 right parapolar cell ; dd, the two dorsal ; e. the ven- 

 tral body cell. 



Fio. IV. 



Dicyemennea eledones, Wagen. (after 

 Whitman). The cap seen from in 

 front, showing the five metapolars 

 (6), of which three are dorsal and 

 two ventral ; a, propolar cell ; D, 

 dorsal, V, ventral surfaces. 



The first trunk cells lie one on each side, and as they differ somewhat 

 from the rest, have been termed " parapolar " cells. The next two are 

 dorsal and ventral, the next couple are lateral, and so on throughout the 

 body, though they are arranged somewhat spirally. The two terminal 

 cells differ slightly from the rest There are only some twelve to twenty 

 trunk cells, exclusive of the parapolara 



The contents of the cells, at first finely granular, soon exhibit in 

 some cells coarser granules, and even crystalloid grains that may coalesce 

 to form balls, which collect in these cells, and as they accumulate they 

 form more or less pronounced swellings, or finally stalked sacs (Fig. 

 II. v) ; these " verruciform " cells never exceed six. The nature of their 

 contents is uncertain ; they are neither fat nor lime, and do not take 

 stains. 



Within the ectoderm is a single "axial cell," from which, at an 

 early stage in the life-history, germ cells are produced in such a way 

 that they lie within the axial cell The latter is derived from the larger 

 of the first two blastomeres, but remains inactive till it has been sur- 



= nine. 



