SPONGID^. 



39 



In the centre of each, surrounded by a clear space 

 may be noticed the * germinal vesicle/ and mthin 

 the latter a minute * germinal spot ' may sometimes 

 he seen. 



9. Ilevelopment. — The development of the 

 Sponges is effected, 



a. by ova and spermatozoa; 



b. by various other bodies, the true nature of 



which is not yet sufficiently determined. 



a. True reproduction has hitherto been proved 

 to take place in Tethya alone, although from 

 analogy, there can be little doubt that it must 



Fig. 8. 



Structure of seed-like body of Spongilla : — a, one of the seed- 

 like bodies of Spongilla Meyeni, shown in magnified section ; b, 

 one of its spicula seen in profile ; c, the same, viewed endways ; 

 d, germs of cells of a (very much magnified) ; e, one of the cells 

 cf a, containing germs ; /, portion of coriaceous membrane of 

 a, showing hexagonal divisions and transparent centres. 



also occur in most other forms of the group. In 

 the fresh-water Sponges small moving corpuscles, 



D 4 



