142 SPONGES 



Keratosa, included foreign bodies are always absent in the fibres 

 of the dendritic type of skeleton ; on the other hand, they are 

 commonly present in the fibres of the reticulate type, a difference 

 perhaps due, as already suggested above (p. 43), to the fact that 

 the former grow originally from the base of the sponge, "while the 

 latter, on the contrary, have, from the first, their growing points at, 

 or near, the upper surface of the sponge body. As regards the 

 amount of foreign bodies taken up by different sponges, a complete 

 series of gradations can be traced. Starting from forms which, like 

 the common bath sponge, have no foreign bodies at all, or only a 

 few, in their principal fibres, we find others in which the amount 

 contained in the principal fibres is greatly increased, the connecting 

 fibres, however, still being free from them ; in others again, both 

 principal and connecting fibres are loaded with foreign bodies 

 (Fig. 94). Finally, the whole skeleton appears to be made up of 

 sand grains and similar particles, between which the spongin can 

 scarcely be made out. In fact, in many of these so-called arenaceous 

 sponges the presence of any spongin at all in the skeleton is 

 disputed. 



Thus in Psammopemrna, an extreme type, the skeleton is made up of 

 isolated sand grains, which are stated to be coated each by a thin cuticle 

 (Marshall) composed of spongin (Polujaeff), and to be united one to another 

 by thin strands of the same substance (Lendenfeld). Haeckel, however, 

 denies the existence of any spongin connecting the sand grains, and has 

 founded a new family, Psamminidae, characterised by a skeleton of 

 foreign bodies without any spongin, for the genus Psammopemma and its 

 allies. 



Two aberrant types of spongin skeleton have been described by 

 Haeckel (1889). In his genus Cerelasma, placed by him amongst the 

 Sponyeliidae, the skeleton is described as consisting of thin epongin 

 lamellae, which branch and anastomose to form a reticular framework. 

 In the meshes of the skeleton are lodged numerous foreign bodies, each 

 as a rule enveloped in a thin coating of spongin. In Haeckel's family 

 Stannoniidae the skeleton is said to be composed of thin fibrillae of 

 spongin, which may branch but do not anastomose, and between which 

 numerous foreign bodies lie in the gelatinous ground substance. Grave 

 doubts attach, however, to the nature both of Cerelasma and the 

 Stannomidae, and it is very probable that they are not sponges at all 

 (see p. 154). 



There remain for mention, finally, the peculiar filaments found in 

 certain genera (Hircinia, Stelospongus, etc.), combined with a spongin 

 skeleton of the ordinary type. Each filament is a long slender twisted 

 thread, slightly thicker in the middle than towards the extremities, and 

 terminating at each end in a knob. The form has been aptly com- 

 pared to that of an ordinary skipping-rope, with pear-shaped handles. 

 Each filament has a thin sheath enclosing a softer medulla, traversed 

 from end to end by an axial thread. The greatest uncertainty prevails 



