Ill 



PHYLUM AND CLASS PORIFERA 



121 



cylinder or vase, may be looked upon as an asexual mode of repro- 

 duction by budding. In some cases asexual multiplication also takes 

 place by the production of external buds ; in others of internal buds 

 in the shape of groups of cells called gemmules, which eventually 

 become detached and develop into new individuals. In the Fresh- 



A.EusjDongia 



B. Pachychalina 



FIG. 90 Microscopic structure of the skeleton in various sponges. A, Euspongria. network 

 of spongin fibres; B, Pachychalina, spongin strengthened by siliceous spicules; C, 

 Spongelia, spougin strengthened by various foreign siliceous bodies, fragments of spicules 

 of other sponges, &c. (After Vosmaer.) 



water Sponges (Spongillidce) multiplication takes place very actively 

 by means of such gemmules, each of which is a spherical group of 

 cells enclosed in an envelope composed of peculiarly shaped siliceous 

 spicules, termed amphidiscs (Fig. 91, right side). These gemmules 

 are formed in the substance of the Sponge towards the end of the 



