SPONGES.-HALLMANN. 



263 



make their appearance, which become more distinct as 

 the surface is approached and terminate each in a few pro- 

 jecting spicules. In longitudinal section, perpendicular to the 

 sponge lamina, these fibres are seen to be arranged pinnately 

 with regard to the mid-line of the section ; and since they usu- 

 ally lie closely, and are then connected by short (mostly 

 unispicular) transverse fibres, the skeleton in its superficial 

 portions becomes for the most part rectangular in mesh. Thus, 

 the pattern of the skeleton changes in passing outwards from 

 the mid-plane to the surface, from more or less irregularly 

 "isodictyal" to more or less "scalariform. " The isodictyal 

 pattern of the mid-region is, however, in most sections, to 

 some extent obscured or interrupted by the dense spicular ag- 

 gregations constituting the "veins" above referred to. 



In older portions of the sponge, the skeleton presents a quitfe 

 different aspect, owing mainly to the presence, in the mid- 

 region of the lamina, of a dense plexus of stout horny fibres. 

 These appear to be quite analogous to the primary fibres of 

 such species as Clathria arcuophora and C. frondosa, and ac- 

 cordingly might be so designated, although they are subse- 

 quent in order of formation to those which have been termed 

 ''secondary" or excurrent fibres. In contrast with the latter 

 fibres, which are pale-coloured, the primary fibres are of a 

 deep yellowish tint and are generally aspiculous : as the latter 

 fact indicates, they arise, for the greater part at least, inde- 

 pendently of the "isodictyal" spicular meshwork occupying 

 the same region. Apparently, they are earliest developed in 

 connection with the radial venations above referred to — the 

 intermediate belts remaining for some time unaffected ; ulti- 

 mately, however, the plexus which they form extends in the 

 mesial plane continuously. In sections from the marginal 

 region, it is seldom that primary fibres are suflSciently 

 developed to be distinguished ; but such sections often show, 

 in the place of these, a yellowish colouration suggestive of 

 diffused spongin. 



The different appearance of the skeleton (as seen in longi- 

 tudinal section perpendicular to the lamina) in the older 

 portions of the sponge, is due also to the increased width (con- 

 sequent upon the growth in thickness of the lamina, and in 

 length of the secondary fibres) of the rectangularly-meshed 

 superficial layer extending between the mesial or "plexal" 

 region and the surface ; and, furthermore, the secondary fibres 

 and their connectives are here provided with a well-developed 

 sheath of spongin. In the older parts of the sponge, therefore, 

 there is nothing in the character of the skeleton to warrant the 

 statement (which Dendy has made) that "The species is in- 

 teresting because it shows a structure intermediate betwec 



