iv PHYLUM CCELENTERATA 139 



t.ikt-n at right angles to its long axis, a large number of radii — about twenty- 

 four — can be drawn from the centre outwards, all passing through similar parts, 

 i.e. along the axis of a tentacle and through similar portions of the body and 

 manubrium. But in the medusa (B) the case is different. The presence of the 

 four radial canals allows us to distinguish four principal radii or per-radii. Half 

 way between any two per-radii a radius of the second order, or inter-radius, may 

 be taken ; half way between any per-radius and the inter-radius on either side a 

 radius of the third order, or ad-radius, and half way between any ad-radius and 

 the adjacent per- or inter-radius, a radius 0/ the fourth order, or sub-radius. Thus 

 there are four per-radii, four inter-radii, eight ad-radii, and sixteen sub-radii. 

 In Obelia the radial canals, the angles of the mouth, and four of the tentacles are 

 per-radial, four more tentacles are inter-radial, and the remaining eight tentacles, 

 bearing the lithocysts, are ad-radial. The sub-radii are of no importance in this 

 particular form. 



Reproduction. — In the description of the fixed Obelia-colony 

 no mention was made of cells set apart for reproduction, like 

 the ova and sperms of a sponge. As a matter of fact, such sexual 

 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. 99 and 100, 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 int» 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 noticedthat the gonad has the 

 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. 103, 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- 

 weed, &c, fixes itself by one end (K), and becomes converted into 

 a hydrula or simple polype (L, M), having a disc of attachment at 

 its proximal end, and at its distal end a manubrium and circlet of 

 tentacles. Soon the hydrula sends out lateral buds, and, by a 

 frequent repetition of this process, becomes converted into the 

 complex Obelia-colony with which we started. 



This remarkable life-history furnishes the first example we have 

 yet met with among the Metazoa of alternation of generations, or 



