42 SPONGES 



chemical formula has been estimated at C3,.H^,;X,,Oj3 (Knikenberg). 

 According to Huiideshagen,^ however, some spongin contains a 

 considerable percentage of iodine, while other varieties contain 

 chlorine and bromine. The iodine containing variety — "iodo- 

 spongin " — yields tyrosin when heated with HoSO^. 



Spongin, as a skeletal element, occurs in two distinct forms ; 

 first, as a cnticular secretion of a tenacious but elastic cementing 

 substance which glues siliceous spicules together into a more or 

 less definite system of skeletal fibres ; and, secondly, in the form of 

 minute elastic fihrillae, secreted within cells, and furnishing a tissue 

 which may be compared to the elastic tissue of higher animals. 

 By atrophy of the spicules in the first case we obtain fibres of pure 

 spongin, as in the so-called horny sponges (see below, p. 139). 



A remarkable property possessed by the spongin fibres of many 

 sponges is that of taking up foreign particles of various kinds into 

 their interior. Sand grains, sponge spicules, Radiolarian or Fora- 

 miniferan skeletons, and such like bodies which fall on to the surface 

 of the sponge body, become included in the fibres, apparently by 

 adhering to the tip of the fibre at its growing point, where it is 

 continuous, in all probability, with the external cuticle of the sponge 

 bod)'. The absorption of foreign particles into the spongin fibre is 

 therefore not so much a question of their travelling down into it, 

 as of their being passively surrounded by spongin as the fibre grows 

 upwards. The filjrcs may be so laden with saml grains and foreign 

 bodies that the skeleton appears made up of them, and the spongin 

 is scarcely visible. Sponge skeletons of this kind are termed 

 arenaceous. The habit of fortifying the skeleton in this way is one 

 which has been acquired independently by forms of diverse affinities, 

 and is perhaps to be regarded as a specialisation, as it were, of a 

 frequent tendency to form a false skeleton by inclusion of foreign 

 particles in the growing sponge body. 



Spongin originates as a secretion of certain cells of the dermal 

 layer termed spongoblasts, which by their discoverer, Schulze, were 

 regarded as belonging to the connective-tissue system, but are now 

 more generally regarded as derived directly from glandular cells of 

 the external flat epithelium. The spongin fibres are formed as a 

 cuticular secretion of the spongoblasts, a fact which explains not 

 only the great similarity, if not identity, in chemical composition 

 that appears to exist l)t'tween the sujjerficial cuticle of many sjxjnges 

 and the spongin of their skeleton, but also the fact that the two 

 may be directly continuous {SpongUla, Evans). The primarily 

 cuticular nature of spongin skeletons further renders intelligible 

 the frequent occurrence of a basal plate of sj)ongin, serving for the 

 attachment of the sponge, especially in sponges belonging to groups 

 {e.g. Clavuliiiti) in which a spongin skeleton is usually absent. In 

 ' Quoted from Lendeufeld, Zoological Record, 1895. 



