230 



ZOOLOGY 



SECT. 



both sexes are formed together in the same plasmodium. In some forms the 

 sexes are united. The sexual animals, especially the females (Fig. 182), 

 bear a considerable resemblance to the Dicyemidae, but instead of the axial cell 

 there are a number of cells, the ova or sperm-cells. The outer cells are 

 arranged in segments or rings. In front is usually a region composed of a few 

 rings in which the outer cells bear cilia which are directed forwards : then 

 comes a shorter region devoid of cilia, and behind that is the longest region, 

 having cilia directed backwards. In shape the body is usually spindle-like 

 the males (Fig. 181) differing somewhat from the females. In about the 

 middle of the internal space of the male is the compact oval testis containing 

 small tailed sperms. Beneath the outer layer in the male, but not in the female, 

 is a layer of fibres sometimes regarded as muscular. The plasmodia multiply 



FIG. 181. Rhopalura giardii, male. 

 (From Bronn's Thierreich, after Julin.) 



FIG. 182. Rhopalura giardi, female. 

 (From Bronn's Thierreich, after Julin.) 



by fragmentation. The development of the embryos either goes on in the 

 intact plasmodium, or the latter breaks up and the embryos are to be found at 

 various stages free in the host. 



In the development of a male from the germ-cell the first segmentation is 

 unequal. The further segmentation results in the formation of a solid morula. 

 The outer cells become differentiated into two distinct groups, the one giving 

 rise to the external layer of the anterior region, the other to that of the 

 posterior region of the body. The inner cells multiply and give rise to the 

 numerous small spermatocytes of the testis. The formation of the layer of 

 fibres only takes place later. 



