120 



THE MICROSCOPE AND ITS REVELATIONS. 



FIG 353. 



portant condition of that flexibility and compressibility on which its uses 

 depend. When spicules exist in connection with such a skeleton, they 

 are usually either altogether imbedded in the fibres, or are implanted into 

 them at their bases, as shown in Fig. 352, A. But smaller and simpler 

 Sponges, such as Grantia, have no horny skeleton; and their spicules are 

 imbedded in the general substance of the body (Fig. 351, B). 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 division in which they are 

 composed of carbonate of lime. The long needle-like spicules (Fig. 

 353), which are extremely abundant in several Sponges, lying close to- 

 gether 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. 488, H). When the spicules project from the horny frame- 

 work, they are usually somewhat conical in form, and their surf ace is often 

 beset with little spines, arranged at regular intervals, giving them a jointed 

 appearance (Fig. 352, A). Sponge-spicules frequently occur, however, 



under forms very different from 

 the preceding; some being short 

 and many-branched, and the 

 branches being themselves very 

 commonly stunted into mere tu- 

 bercles (some examples of which 

 type are presented in Fig. 488, A, 

 c); whilst others are stellate, hav- 

 ing a central body with conical 

 spines projecting from it in all 

 directions (as at D of the same 

 figure). Great varieties present 

 themselves in the stellate form, 

 according to the relative predomi- 

 nance of the body and of the rays: 

 in those represented in Fig. 353, the rays, though very numerous, are ex- 

 tremely short; in other instances the rays are much longer, and scarcely 

 any central nucleus can be said to exist. The varieties in the form of 

 Sponge-spicules are, in fact, almost endless; and a single Sponge often pre- 

 sents two or more (as shown in Fig. 353), the stellate spicules usually occur- 

 ring either in the interspaces between the elongated kinds, or in the exter- 

 nal crust. 1 The spicules of Sponges cannot be considered, like the r aphides 

 of Plants ( 359), simply as deposits of Mineral matter in a crystalline 

 state; for the forms of many of them are such as no mere crystallization 

 can produce; they generally (at least in the earlier stage of their forma- 

 tion) possess internal cavities, which contain organic matter; and the 

 calcareous spicules, whose mineral matter can be readily dissolved away 

 by an acid, are found to have a distinct animal basis. Hence it seems 

 probable that each spicule was originally a segment of sarcode, which has 



1 A minute account of the various forms of spicules contained in Sponges is 

 given by Mr. Bowerbank in his First Memoir ' On the Anatomy and Physiology 

 of the Spongiadse, 'in "Philos. Transact.," 1858, pp. 279-332; and in his "Mono- 

 graph of the British Spongiadae" published by the Ray Society. The Calcareous 

 Sponges have been made by Prof. Haeckel the subject of an elaborate Monograph, 

 "Die Kalkschwamme," Berlin, 1872. 



Siliceous Spicules of Pachymatisma. 



