SPONGES.- KALLMANN. 221 



firm and tough, and of compact texture ; the younger portions 

 are fine-textured, and, in the dry sponge, of almost silky soft- 

 ness. The colour in alcohol is dark brown ; in the dry state 

 it varies from greyish yellow to brown. 



As seen under moderately low powers of the microscope, 

 the main skeleton, except in the vicinity of the surface of the 

 sponge, bears a striking resemblance to that of a Chaliuinc 

 sponge. This is mainly due to the comparatively small size 

 and only moderate abundance of the spicules ; to the aggrega- 

 tion of the intrafibral spicules into a well-defined axial core ; 

 and to the fewness of the echinating acanthostyles. Generally 

 speaking, the skeleton reticulation consists chiefly of closely 

 approximated and frequently inosculating longitudinal main 

 fibres, which contain a somewhat meagre, fairly compact 

 spicular core, and are joined at irregular intervals by short 

 usually unispicular connecting fibres. 



In the superficial parts of the sponge-branches, however, 

 the pattern of the skeleton is of a somewhat different char- 

 acter. Thus, the "excurrent" main fibres {i.e., those which run 

 obliquely outwards to the surface of the branches) become 

 somewhat widely separated as the surface is approached, and 

 between them the connecting fibres form an interreticulation 

 or plexus. Moreover, the spicules of these main fibres become 

 reduced in number and more dispersed, whilst those of the 

 connecting fibres increase in number. The distinction between 

 main and connecting fibres consequently tends to become 

 obscure, and the general pattern of the skeleton approaches 

 that which is characteristic rather of Myxilla. It is to be noted 

 also that the fibral spicules (principal styli) of this portion of 

 the skeleton are of distinctly stouter proportions than those 

 which core the main fibres of the interior. 



Irrespective of this contrast between its inner and outer por- 

 tions, the skeleton may yet present quite different appearances 

 in different parts of the sponge, owing to disparity in the stout- 

 ness of fibres. Thus, in the denser parts — especially in the 

 internodal regions of the branches — the fibres are composed 

 chiefly of spongin and may attain a diameter of 80 jt ; whilst 

 in the younger portions of the sponge — especially in the nodal 

 regions of the branches — spongin is much reduced in amount 

 — often to such an extent that it forms but a scarcely percep- 

 tible sheath enclosing the spicules — and even the stoutest 

 fibres may be less than 20 ;t in diameter. There is in this 

 connection a further difference also, due to the fact that those 

 fibres which are deficient in spongin likewise show a reduction 

 in the number of their axial spicules. In regions of greater 

 density, accordingly, the inner or deeper skeleton is a close 



