228 



ZOOLOGY 



SECT. 



body. The form of the calotte varies a good deal, according to age ; in young 

 specimens it is isotropic (i.e., symmetrical around the long axis) ; in the adult 

 condition ventral and dorsal sides are distinguishable. It consists of a swollen 

 disc of four cells and a ring of four or five pole cells. The cells of the head all 

 bear cilia, which are shorter and thicker than those of the body-cells. 



The body consists of a single large axial cell, and of a single layer of outer 

 cells which completely invest the axial cell. The outer cells which follow 

 immediately on the head are distinguishable from the rest by their granular 

 contents, and by their being dilated internally in such a way that the apex of 

 the axial cell is constricted. 



The axial cell is either almost completely cylindrical or spindle-shaped, and 

 is covered in its entire extent by the outer cells. It presents a differentiated 



FIG. 178. Dicyema paradoxum, 



with infusoriform embryos (males). 

 (From Bronn's Thierreich, after 

 Kolliker.) 



FIG. 179. Dicyema paradoxum, 



with vermiform embryos. (From 

 Bronn's Thierreich, after Kolliker.) 



cortical layer, beneath which the finely granular gelatinous contents are at 

 first homogeneous, but afterwards become vacuolated. In the middle of the 

 cell is a large oval or ellipsoidal nucleus. 



The life-history is a true alternation of generations. The primitive nucleus 

 of the axial cell divides mitotically to form a smaller asexual germ-nucleus and 

 a larger nucleus the definitive nucleus (somatic nucleus) of the axial cell. 

 Further germ-nuclei result from subsequent divisions. The germ-cells undergo 

 a process similar to segmentation. Of the cells thus formed one gives rise to 

 the axial cell of the embryo : the others, increasing in numbers and becoming 



