lO 



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. 

 Eilhitnlia Schuhei, Pol. (After PoU-jaeff, 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 



A. 



B 



Yir.. 14. 



A, yerticilUtes anastomans, yVSint. (After Zittel.) B, Pttrostroma Schulzci, Dixl. 

 (After Doderleiii.) 



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- 



