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MANUAL OF ZOOLOGY 



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. 40, right side). All sponges multiply by a sexual 

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



FIG. 40. Various forms of sponge spicules. (From Lang's Text-Book.) 



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 



