Chap. IX.j 



SPICULES OF SPONGES. 



279 



sponges), or the organic substance becomes impregnated 

 with calcareous salts (calcareous sponges), or with 

 siliceous (siliceous sponges). Considerable variations 

 are, moreover, to be seen in the extent to which this 

 impregnation takes place, so that while the fresh- water 

 sponge (Spongilla) has but few and simple siliceous 

 spicules, the Lithistidse are quite hard and strong. 

 In most cases the 

 inorganic skeleton is 

 spicular, and not 

 continuous ; but in 

 some, as " Venus's 

 Flower Basket : ' 

 (Euplejtella), a deli- 

 cate framework of 

 siliceous particles is 

 left after all the or- 

 ganic material has 

 been removed (Fig. 

 114). 



The spicules vary 



i i_i Fi g. 114. Section through the Wall of 



considerably in form, Euplecteila ( x 75). 



being uniaxial Or p, Pores; -f, flagellated chambers. (After 



needle-shaped, tri- 

 axial (this is the cha- 

 racteristic form in the Calcispongiae), or quadriaxial ; 

 connected with these are bi, tri, quadri, and sex- 

 radiate spicules, which may by the loss of some, and 

 the greater development of other rays, take on the 

 most different shapes. Some spicules are multi-radiate, 

 and others curved. Some project beyond the body of 

 the sponge, as in the glass-rope sponge (Hyalonema ; 

 Fig. 115), where anchoring spicules as much as 

 eighteen inches long have been observed. In addi- 

 tion to these proper skeletal spicules, others which 

 are smaller take an important part in giving firmness 

 to the sponge body, and even, as in the case of tho 



