TETRACTINELLIDA 215 



each family but not in others. Thegenus Pachyraatisma, of which 

 we have the species P. johnstonia and P. normani in these 

 islands, exemplifies this ; it belongs to the highly differentiated 

 family Geodiidae, possesses an elaborate cortex with chones, but 

 its main skeleton is non-radiate. 



Disyringa dissimilis is remarkable for the perfection of its 

 symmetry, and for the absence of that multiplication of parts 

 which is so common among sponges. It possesses a single 

 inhalant tube and a single osculum (Fig. 107). Until quite 

 recently it stood alone in the restriction of its inhalant apertures 

 to a single area. Kirkpatrick, however, has now described a 

 sponge — Spongocardiiom gilchristi ^ — from Cape Colony, in which 

 the dermal ostia are concentrated in one sieve -like patch 

 at the opposite pole to the single osculum. Disyringa is still 

 without companions in the possession of an inhalant tube. The 

 concentration of ostia into sieve areas occurs again in Cinachyra, 

 each sponge possessing in this case several inhalant areas with 

 or without scattered ostia also. 



Order II. Lithistida. 



The characteristic spicule of Lithistida — the desma — may be 

 a modified calthrop (tetracrepid desma), or it may be produced 

 by the growth of silica over a uniaxial spicule (rhabdocrepid 

 desma) (Fig. 110, q), or it may be of the polyaxon type. It is 

 probable that the group is polyphyletic,^ and that some of its 

 members should remain associated with Tetractinellida, while others 

 should be removed to Monaxonida. Forms with tetracrepid desmas, 

 and those forms with rhabdocrepid desmas which possess triaenes, 

 have Tetractinellid affinities, while forms possessing rhabdocrepid 

 desmas but lacking triaenes, and again those in which the desmas 

 are polyaxon, are probably descendants of Monaxonida. 



Owing to the consistency of the skeleton Lithistida are 

 frequently found as fossils. The commonest known example is 

 Siphonia^ As in the case of so many other fossil sponges the 

 skeleton is often replaced by carbonate of lime, a fact which 



1 Marine Investigations in South Africa, i. 1902, p. 224. 



2 Cf. Sollas, Encydopaidia Britanniea, 1887, art. "Sponges," and Sohramraen, 

 Mitth. Mus. Hildesheim, 14, 1901. 



» Sollas, Quart. Journ. Geol. Soc. xxxiii. 1877, p. 790. 



