in PHYLUM PORIFERA 89 



consists throughout of siliceous spicules bound together by 

 a siliceous cement. 



Reproduction in the Sponges is effected either sexually or 

 asexually. The process by which, in all but the simplest 

 forms of Sponges, a colony of zooids is formed from the 

 originally simple cylinder or vase, may be looked upon as 

 an asexual mode of reproduction by budding. Asexual 

 multiplication also assumes the form in some cases of a 

 process of production of internal buds in the shape of 

 groups of cells called gemmules, which eventually become 

 detached and develop into new individuals. In the Fresh- 

 water Sponges (Spongillida) 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. 36, right side). All Sponges multiply by a sexual 

 process by means of male cells, or sperms, and female 

 cells, or ova. Ova and sperms are developed in the same 

 Sponge, but rarely at the same time. The cell destined to 

 form sperms divides into a number of small cells, giving 

 rise to a rounded mass of sperms. The latter, when 

 mature, have oval or pear-shaped heads and a long taper- 

 ing appendage or tail. Each cell destined to form an ovum 

 enlarges, and eventually assumes a spherical form. After a 

 sperm has penetrated into its interior and effected impreg- 

 nation, it usually becomes enclosed in a brood-capsule 

 formed for it by certain neighbouring cells, and in this 

 situation, still enclosed in the parent Sponge, it undergoes 

 the earlier stages of its development. Eventually it be- 

 comes free as a ciliated larva, which pursues a free existence 

 for a time, swimming about by the agency of the cilia, till 

 after a time it becomes fixed and develops into the adult 

 form. 



