Till; DICYEMIDA. 



in inner 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 



Fro. 158. IMcyema I. D. tyintf. The large papillae of the cortical layer and the 

 yenns in th<> interior of the axial cell are noticeable. 



II. D. tyjiu*. Dim-rent sn.ues of the development of a vermiform germ. 



III. InfuHoritbnn embryo found free in the renal organs of Eledone mosc/iata, treated 

 with osmicacid: p, the urn: r i, iis capsule; s, its lid; i, multiimcleate cells in its 

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



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

 Strongly-ref racting globules and rods accumulate in some of 

 the ectodermal cell?, and cause them to project in the form 

 of papillae. 



The axial cell is a mass of protoplasm. Its relatively 

 dense outer layer 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 embrvo 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 Nematogena* 

 while the others are named Rhombogena. 



In the Nematogena, 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 1 ; to an axial cell, around which the other cells 

 arrange themselves, until eventually they inclose it. Before 



