HIGH SCHOOL ZOOLOGY. 



251 



or even of parts of the living 

 sponge, may also give rise to 

 them. A peculiar kind of bud- 

 ding occurs in the fresh-water 

 Sponges, which recalls the form- 

 ation of the winter-buds of the 

 Polyzoa, as it takes place under 

 similar conditions. The buds 

 are called statoblasts or " gem- 

 mules " and are protected by 

 characteristic spicules (Fig. 188), 



10. Fossil I'emains of sponges 

 are abundant in the earlier for- 

 mations, but they reached the 

 heiglit of their development 

 during the upper Secondary 

 period. 



Fig. 187. --Diagram of a Calcareous 

 sponge, with a single osculum. 



a, ectoderm ; 6, mesoderm, with trira- 

 diate spicules ; c, lining of gastro-vascu- 

 lar cavity ; f, ciliated chambers lined with 



"collar-cells^' e osculum The J,nows f found iu shallow water ; the 



indicate the direction of the current in- ' 



wards through the pores, and outwards same is true of the fleshy Non-cal- 



1 1 . The Calcarea are chiefly minute 



through the osculum. 



others attain a considerable 

 size. Some of those with 

 purely siliceous skeletons, 

 like Euplectella and Hyalo- 



carea — Halisarca, — but most of the 



.4. 





/S. 



nema, are most beautiful k J 



objects ; they occur in the ''~~'j 

 depths of the ocean, anchor- l-^^\ 

 ing themselves in soft mud 

 by a wisp of glassy threads. Vf 

 In these foi-ms, there are 

 triaxial spicules in addition 

 to the fibres, but in many 



cjTioTicrps from shallow wnter ^'S''- 188.— Diagram of gemmule of a fresh-water 



sponges trom SUallOW water, gp^^.e, showing the coating of amphidislts and the 



the spicules alone constitute aperture ; b, an amphidisk of Ephydatia- the river 



the skeleton, which may ^1'°"^ * 



