530 



SPONGE-SPICULES. 



Fig. 273. 



Siliceous Spicules of Pachymatisma. 



giving them a jointed appearance (Fig. 272). 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 tubercles (some 



examples of which 

 type are presented 

 in Fig. 403, a, c) ; 

 whilst others are 

 stellate, having a 

 central body with 

 conical spines pro- 

 jecting from it in 

 all directions (as at 

 d of the same figure). 

 Great varieties pre- 

 sent themselves in 

 the stellate form, 

 according to the re- 

 lative predominance 

 of the body and of 

 the rays : in those re- 

 presented in Fig. 273, the rays, though very numerous, are extremely 

 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 presents two or more (as shown in Fig. 273), the 

 stellate spicules usually occurring either in the inter-spaces between 

 the elongated kinds, or in the external crust.* In one curious 

 Sponge desci-ibed by Mr. Bowerbank (the Dusideia fragilis), the 

 spicules are for the most part replaced by particles of sand, of very 

 uniform size, which are found imbedded in the horny fibre. — The 

 Spicules of Sponges cannot be considered, like the Raphides of 

 Plants (§ 293), simply as deposits of mineral matter in a crystalline 

 state. For the forms of many of them are such as no mere crys- 

 tallization can produce ; many of them 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 

 undergone calcification or silicification, and by the self-shaping 

 power of which the form of the spicule is mainly determined. 



407. Of the Reproductive process in Sponges, much has yet to 

 be learned : — the following is perhaps the most probable account 



* 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 Spongiadae,' in "Philos. Transact.," 1858, pp. 279- 

 332 ; and in his "Monograph of the British Spongiadas" published by the 

 Ray Society. 



