56 



SYSTEMATIC ZOOLOGY 



viduals composing it may be distinguished by noting the number 

 of large openings, or oscula, of which one belongs to each ani- 

 mal (Fig. 40). There is another form of budding, which may be 

 called internal budding; it occurs typically in some fresh-water 

 sponges. Some of the mesenchyme cells collect into groups, 



- 



A.Oscaria 



C.Eus^ongia 



B.Psammoclema 



D. Porerion 



Fig. 39. Various tonus of sponges. A, incrusting, with numerous rounded 

 elevations; B, ramifying, subcylindrical ; C, toilet sponge, massive, with broad 

 base; I), Neptune's cup, vase-shaped. (After Vosmaer, from Parker and 

 Haswell's Text-book. 1 



and each group secretes a cyst about itself, forming what is 

 called a gemmule (Fig. 41). The rest of the sponge dies, 

 and after a time the cells emerge from the gemmule and 

 develop into a new sponge. Reproduction also takes place by 

 means of fertilized ova. The germ-cells develop from mesen- 

 chyme cells, and the ova are fertilized while still within the 

 mesenchyme ; there they develop in part, and the young, 



