10 



SPONGES 



the body wall the primitive vase-like sponge person may assume 

 a more massive form, and in the simplest cases becomes barrel- 

 shaped (Fig. 16) or globular (Fig. 30), according to the degree of 



Fio. 13. 

 Eilhardia Schulzei, Pol. (After Polejaeff, Challenger Reports.) Natural size. 



thickening. If the growth predominates at the base of the sponge 

 it acquires the shape of a shallow cone or volcano, the crater being 

 represented by the osculum, and in such forms the vertical height 



B 



FIG. 14'. 



A, Verticillites anastomans, Mant. (After Zittel.) E, Petrostroma Schulzei, Dod. 

 (After Doderlein.) 



may be very small as compared with the horizontal extent, until in 

 extreme cases the sponge becomes a mere crust, spread over the 

 surface to which it is attached, and rising slightly in the region of 

 the osculum. On the other hand, the sponge may become sub- 



