IV 



PHYLUM CCELENTERATA 



219 



illy rounded at the ends, without any trace of bilateral symmetry. The 

 ly is divided into several rings or segments. Beneath the single layer of cells 

 constituting the ectoderm is a layer of muscular fibres. In the centre is a mass 

 of eiidoderm cells. There are distinct male (Fig. 165) and female (Fig. 166) 

 individuals, the former fusiform in shape, the latter oval, cylindrical, or com- 

 pressed. In one of the species there are two different forms of females, the one 

 producing only male young, the latter only female. 



Motile sperms are developed in the interior of a mass of sperm-cells the 

 ffxfis. When mature these pass out between the muscular fibres ; the ectoderm 

 Cecils disintegrate and the sperms es- 

 cape, the animals perishing. In the 

 cylindrical female the conical anterior 

 segment of the body becomes separated 

 off from the rest as a sort of operculum, 

 permitting of the discharge of the ova. 

 In the compressed forms the body be- 

 comes broken up into fragments, in 

 which the ova lie embedded. After 

 impregnation the eggs are developed 

 in the interior of protoplasmic masses, 

 one such mass in some cases containing 

 only male or only female embryos, in 

 other cases both. The male ovum (Fig. 

 167) divides into two cells, a smaller 



Fro. 167. Stages in the development of 

 Rhopalura Giardii. ( From Bronn's 



Tliitrrcich.) 



and a larger ; the latter remains for 

 a time undivided, but subsequently 



segments to form the cells of the eiidoderm. The former at once divides to form 

 a number of small cells, which grow over the large cell to form a continuous 

 ectoderm layer by a process of epiboly. There is some discrepancy between the 

 statements of different authors as to the development of the females ; but there 

 appears to be unequal segmentation followed by epiboly, the peripheral layer 

 of the eiidoderm afterwards' giving rise to the middle fibrous stratum (mesoderm). 



SALINELLA. 



Perhaps having more claim than the Orthonectidae and DicyemidaB to a position 

 intermediate between the Protozoa and the Metazoa is a remarkable infusorian- 





FIG. 10$. Salinella, longitudinal section. (After Frenzel.) 



FIG. l(jy. Salinella, transverse section. (After Frenzel.) 



