140 



ZOOLOGY 



SECT. 



cells are found only in their fully developed condition at least 

 in the medusae. Hanging at equal distances from the sub-umbrella, 

 in immediate relation with the radial canal and therefore per- 

 radial in position, are four ovoid bodies (Figs. 100 and 101, gon), 

 each consisting of an outer layer of ectoderm continuous with 

 that of the sub-umbrella, an inner layer of endoderm continuous 

 with that of the radial canal and enclosing a prolongation of the 

 latter, and of an intermediate mass of cells which have become 

 differentiated into ova or sperms. As each medusa bears organs 

 of one sex only (testes or ovaries, as the case may be), the individual 

 medusae are dioecious. It will be noticed that the gonad has the 



Fio. 104. Stages in the development of two Zoophytes (AH, Laomedea, I M, Euden- 

 drmxn) allied to Obelia ; A F, stages in segmentation ; O, the planula enclosed in the 

 maternal tissues ; H, the free-swimming planula ; / M, fixation of the planula and 

 development of the hydrula. (From Parker's Biology, after Allman.) 



same general structure as an immature zooid an outpushing 

 of the body-wall consisting of ectoderm and endoderm, and 

 containing a prolongation of the enteric cavity. 



Development. When the gonads are ripe, the sperms of the 

 male medusae are shed into the water and carried by currents to 

 the females, impregnating the ova, which thus become oosperms 

 or unicellular embryos. The oosperm undergoes complete seg- 

 mentation (Fig. 104, A F), and is converted into an ovoidal body 

 called a planula (G, H), consisting of an outer layer of ciliated 

 ectoderm cells and an inner mass of endoderm cells in which a 

 space appears, the rudiment of the enteron. The planula swims 

 freely for a time (H), then settles down on a piece of timber, sea- 



