THE DICYEMIDA. 



559 



manner that the head is bilaterally symmetrical. Sometimes 

 the polar cells constitute the whole of the cephalic enlarge- 

 ment ; but, in others, cells of the adjacent part of the body 



Pig. 158.— Dicyema. — I. Z>. typus. The larsre papillae of the cortical layer and the 

 germs in the Interior of the axial cell are noticeable. 



II. 7>. typus. Difl'erent stages of the development of a vermiform germ. 



III. Infusoriform embryo found free in the renal organs of Eledo/te rfwschata. trented 

 with osmic acid: p, the urn; ca, its capsule; s, its lid; i, muliiuucleate cells in its 

 interior. (After Van Beneden, /. c.) 



(parapolar cells) contribute to the investment of the head. 

 Strongly-refracting globules and rods accumulate in some of 

 the ectodermal cells, and cause them to project in the form 

 of papilljB. 



The axial cell is a mass of protoplasm. Its relatively 

 dense outer la3'er passes into a central reticulation, in the 

 midst of which there is a large oval nucleus. 



Reproduction takes place by the formation of germs, and 

 the development of embryo from them, in the axial cell. The 

 embryos are of two kinds, the one vermiform, the other in- 

 fusoriform, and are not met with in the same Dicyema, but in 

 individuals of somewhat different characters. Those which 

 give rise to the vermiform embryos are termed Neinatogena^ 

 while the others are named Mhomhogetia. 



In the N^ematogena, the germs arise in the protoplasmic 

 reticulum of the axial cell, and, at first, are minute spherical 

 bodies, each of which is provided with a nucleus. This germ- 

 cell divides into two, and each of these again becoming bi- 

 sected, four cells are produced, of which one remains undi- 

 vided, while the rest go on dividing. The former enlarges, 

 and gives rise to an axial cell, around which the other cells 

 arrange themselves, until eventually they inclose it. Before 



