136 



SPONGES 



the incurrent folds of the canal system, with their main shafts reaching 

 from the centre to the periphery (Fig. 91). During subsequent growth 



the new spicules, which are 

 formed after the sponge has ex- 

 ceeded a certain size, may in a 

 few instances be disposed irregu- 

 larly, so that the full - grown 

 sponge exhibits no trace of the 

 radiating arrangement, except 

 perhaps close to the outer sur- 

 face ; most usually, however, the 

 spicules formed later retain the 

 radial arrangement, so that the 

 spicule sheaves of the earlier 

 stage are converted into fibres 

 radiating from the centre to the 

 periphery, often with a pro- 

 nounced spiral twist. 



91. The surface of the sponge 



Mode of arrangement of spicules in a young may become " hispid " by the 

 Stellettid sponge, ft***, normal, Soil. (After pro j ection o f ra dially arranged 



spicules beyond the limiting 



epithelium of the body wall, and the " hispidating " spicules may be 

 specially differentiated to form protecting fringes round the openings 

 of the oscula and incurrent canals, or to furnish a root tuft similar 

 to that of some Hexactinellida. A characteristic feature of Tetracti- 

 nellids is the differentiation of a special cortex, which may have 

 a skeleton distinct from that of the pulp, both as regards arrange- 

 ment and composition (cf. Fig. 87). Finally, in those forms in 

 which there is an elongated oscular tube, it is supported by a 

 palisade of special spicules forming a cloacal skeleton. 



The microscleres are found scattered in the parenchyma, and 

 may be sharply differentiated in the two regions of the body, cortex, 

 and pulp. 



Union of tlie Spicules. Spongin is said to be present in 

 minute quantities in some forms, but it never has any appreciable 

 importance, 1 and is practically absent, as also any other form of 

 special cementing substance. The spicules are held together by 

 interlocking and by the fibrous cortex. In Choristida they fall 

 apart when macerated. In the Lithistida, however, the complicated 

 desmas interlock by means of the tubercles or their branches to 

 form a compact skeletal framework which imparts to the sponge a 



1 With the exception of Thymosia, which is described as having a skeleton of 

 spongin fibres radiating upwards from the base. Each fibre is "verrucose," being 

 composed of nodules of spongin agglomerated together, and contains no foreign bodies 

 (Topsent). More evidence seems to be needed as to the true nature of the fibres in 

 question. 



