154 APLYSINA. 



The colour of the spouge wheu preserved iu spirit is dirty copper-red, some- 

 what similar to the colour of spirit-specimens of A. aerophoha. The dry 

 skeleton is dark reddish brown, very hard and stiff, somewhat brittle. 



The sJceleton consists of fibres 0-5-0-7 millim. thick, which extend longitudi- 

 nally upwards in the middle of the digitate branches in the case of the solid 

 forms, or in the wall of the oscular tubes in the hollow ones. They are here 

 pretty close together, 1-1-5 millim. only apart. From them dendritically rami- 

 fying branches are given off, which extend to the surface. The final ramifi- 

 cations of these terminate in the conuli. These fibres are of very varying 

 thickness, some of them are nearly as thick as the longitudinal fibres in the 

 interior. The most slender branches are about 0-16 millim. thick. 



According to Eidley (loc. cit.) the fibres of the Seychelles specimen are not 

 over 0-7 millim. thick, whilst according to Carter {loc. cit.) the fibres of his 

 Australian specimen are over 1 millim. thick. The same is the case with 

 Hyatt's Dendrospongia crassa from Nassau. I agree with Eidley in con- 

 sidering such distinctions insufficent to warrant the establishment of separate 

 species. 



The pith forms, in the specimen examined by me, ^ of the fibre. In 

 Hyatt's fig. 1 (loc. cit.) of Dendrospongia crassa an axial canal -^ of the thick- 

 ness of the fibre is represented. The width of these canals, which always make 

 their appearance iu the dry skeleton, often depends on the degree of desiccation, 

 and cannot always be considered identical with the thickness of the axial 

 pith-cylinder in the fibres of the living sponge. The width of the meshes of 

 the network formed by the irregularly anastomosing fibres is 1-3 millim. 



According to Carter {loc. cit.) the spongin cortex of the fibres consists of 

 loosely-jointed, concentric, cylindrical layers, and cells are said by him to occur 

 in the outermost layers. I have not seen any cells of this kind in the speci- 

 mens examined by me. Eidley {loc. cit.) describes abundant granular cells in 

 the skin. These measure 0*013 millim. and are colourless in the Australian 

 specimens, and measure only 0-008 millim. and are brown in the specimen 



from the Seychelles Islands. 



The ciliated chambers are very abundant in the interior and exceedingly 



small ; they measure only 0-026 millim. in diameter. 



This species shows in the structure of its skeleton some similarity with 



Dendrilla, a hollow-fibred sponge of another order. Particularly the dendritic 



mode of ramification of the fibres is similar to that of Dendrilla. The small 



spherical ciliated chambers, however, show clearly that it is a true Aplijsina. 

 Why Hyatt {loc. cit.) has placed " Aplysina aerophoha, 0. Schmidt, pars," 



as a synonym of Dendrospongia crassa, I do not know ; they are very 



different. 



Geographical Distkibution. — Atlantic : Nassau {Boston Society Coll.). 



