Elements of the Spongida. 351 



nexions) retracts its cilium and bodily falls down into the form 

 of a creeping amoeboid cell, the latter may be a spongozoon 

 which, migrating in this form through the plastic substance of 

 the sponge, carries about with it the ovum, and thus becomes 

 one of the so-termed " amoeboid " or wandering cells of the 

 sponge. Hence the origin of the ovum in one of these may 

 be conjecturally explained. But this is not always the case, 

 as will presently be seen, from which and additional instances 

 of a similar kind we shall learn that, in form, location, and 

 composition, the germinal elements of the sponge may vary 

 more or less with the species to which they belong. 



After the ovum has gone through the segmentary stage it 

 passes from its location in the substance of the sponge into the 

 neighbouring excretory canal, where Grant, in 1826, de- 

 scribed and represented it in Ualichondria panicea, Bk., in the 

 following words : — " By examining the sponge carefully with 

 the microscope we are surprised to find that many of the 

 mature ova are now hanging by their tapering extremity from 

 the parietes of the internal canals " [op. et loc. cit. p. 129, pi. ii. 

 fig. 26) ; all of which was verified in Tethea zetlandica, Cart., 

 in 1872 (' Annals,' vol. ix. p. 426, pi. xxii. figs. 14 and 15). 



After this the ovum, now in its embryonic state, gets out 

 into the water through one of the vents on the surface of the 

 parent sponge. 



In the Calcisponges the ovum is first seen in the substance 

 which fills the interval between the chambers or ampullaceous 

 sacs, that is, outside the latter ; passing afterwards, as it as- 

 sumes the embryonal form, into the chamber itself, and thence 

 through the inner aperture of the chamber into the cavity of 

 the cloaca, whence it finally issues through the orifice of 

 this cavity, also into the water. 



Returning to the ovum in the Carnosa or fleshy sponges, I 

 fortunately have found among Mr. Wilson's Australian 

 specimens an ovigerous one of Chondrosia sjnirca^ viz. that 

 to which I have alluded in the ' Annals ' of 1886 (vol. xviii. 

 p. 274), and again in the present year (vol. xix. p. 288), in 

 which the ova, of which there is a single one of considerable 

 size in several of the ampullaceous sacs respectively, are not too 

 small to escape being recognized, nor too large to have destroyed 

 all trace of these sacs by having extended beyond their limits 

 (see woodcut, p. 352) ; thus the sac, wiiich is elliptical in form, 

 averages 14-6000ths in, by ll-6000ths in. in its greatest dia- 

 meters, and the ovum^ which is also elliptical, 8-6000ths by 6- 

 6000ths in. in its greatest diameters; so that the hitter is a little 

 more than half the size of the former. Each ovum consists of a 

 capsule closely applied to the sac of the granular yolk on one 



