454 



FOEAMINIFEKA, POLTCYSTINA, AND SPONGES. 



flexibility and compressibility on which its uses depend. When 

 spicules exist in connection with such a skeleton, they are either 

 altogether imbedded in the fibres, or they are implanted into 

 them at their bases, as shown in Fig. 218. In the curious and 

 beautiful Bictyochalix pumiceus of Sarbadoes, however, the en- 

 tire network of fibres is composed of silex, and is so transpa- 

 rent that it looks as if composed of spun glass. There are many 

 Sponges in which no fibrous network can be discerned, the 

 spicules lying imbedded in the midst of the sarcode mass ; such 

 is the case in G-rantia (Fig. 217, a), whose triradiate spicules are 

 composed of carbonate of lime. Sponge- spicules are much more 

 frequently siliceous than calcareous ; and the variety of forms 

 presented by the siliceous spicules is much greater than that 

 which we find in the comparatively small number of species' in 

 which they are composed of carbonate of lime. The long 

 needle-like spicules (Fig. 219) which are extremely abundant in 

 several Sponges, lying close together in bundles, are sometimes 

 straight, sometimes slightly curved ; they are sometimes pointed 

 at both ends, sometimes at one only ; one or both ends may be 

 furnished with a head like that of a pin, or may carry three or 

 more diverging points, which sometimes curve back so as to form 

 hooks (Fig. 334, h). When the spigules project from the horny 

 framework, they are usually somewhat conical in form, and their 

 surface is often beset with little spines, arranged at regular 

 intervals, giving them a jointed appearance (Fig. 218). Sponge- 

 spicules frequently occur, however, under forms veiy different 

 from the preceding ; some being short and many-branched ; and 

 the branches being themselves very commonly stunted into mere 



tubercles (some examples of 

 Fig- 218. which type are presented in Fig. 



334, a, c) ; whilst others are stel- 

 late, having a central body with 

 conical spines projecting from it 

 in all directions (as at d of the 

 same figure). Great varieties 



Fia. 219. 



FijT. 218. Portion of Malichondrm (?) from Madagascar, with spicules projecting from the fibrous 

 network. * 



Fig; 219. Siliceous Spicules of Pactiymatisma. 



present themselves in the stellate form, according to the relative 

 predominance of the body and of the rays ; in those represented 



