314 



A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



and connecting fibres cannot be distinguished. They are all alike 

 as in Carter's (1) genus Coscinoderma, recently also described by 

 Polejaeff. (2) 



The fibres are circular and cylindi-ical^ and they have a diameter 

 of 0-02 mm. The regular network formed by them is pretty 

 loose. In the true body of the sponge itself the meshes average 

 0.2 mm. and in the vestibule tissue 0-5 mm, 



At the joining points of the fibres, in the true body of the 

 sponge only, we find sandy granules of uniform size, 07ie in each 

 joining jioint. (Plate XXXIL, fig. 21, Plate XXXIII., fig. 22.) 

 These measure 0-14 mm. in diameter on a average and are of a 

 more or less spherical shape. Elongate sand grains seem never to 

 occur. 



These sand grains, which form, as it will be seen from the 

 above, an integral part of the whole skeleton, are enclosed in a 

 horny coating (Plate XXX., fig. 17), about half as thick as the 

 fibres which originate from it. This coating is stratified ; but the 

 layers are not very clearly visible. 



The presence of these sand grains is rendered particularly 

 remarkable by the fact, that no sand whatever occurs in the outer 

 skin of the sponge. As a rule we find in the skin of such 

 sponges which contain foreign bodies in their fibres, also similar 

 foreign bodies, in the skin. 



There can be no doubt that these sand grains are originally 

 attached to the tips of the conuli, and from thence they 

 api^arently wonder centripetally because they actually remain in 

 the same place whilst the sponge is growing and the conuli extend 

 beyond them. They are then sought by the growing horny fibres 

 and retained in their joining points. 



(1) H. J. Carter. Conti-ibutions to our Knowledge of the Spongida. 

 Annales and Magazine of Natural History. vSeries ,5, X^ol. XII., p. 309. 



(2) N. Polejaeff', Report on the Keratosa. The Zoology of H.M.S. 

 Challenger. Part XXXI., p. 28. 



