STRUCTURE AND ECONOMY OF TETHEA. 407 



the surface from which the spicula project in greatest abund- 

 ance. The spicula are arranged in great numbers along the 

 margins of the grooves, projecting obliquely over them from 

 their opposite margins, like palisades along the faces of a 

 ditch. The fissures, unlike the grooves, are situated in the 

 fleshy and apparently more recently-formed portions of the 

 surface. 



The character of the perforated grooves associates them 

 with the oscula or feculent orifices of the typical sponges. 



On tearing a specimen through the centre, the arrange- 

 ment of the nuclear radiating structure and veins peculiar 

 to Tethea may be observed. The central nucleus is not 

 so apparent — fleshy substance, with small areolar spaces, 

 predominating over the spicular structure. The radiating 

 masses of fleshy substance, with their bundles of spicula, are 

 well marked. Numerous canals converge between the radiat- 

 ing columns towards the centre, gradually degenerating, by 

 frequent anastomoses, into freely communicating areolar 

 spaces. Other canals of larger size, and lined by a semi- 

 opaque membrane, collect obliquely across the radiating 

 masses towards the perforated grooves on the surface. 



The peripheral rind peculiar to Tethea does not exist over 

 the whole surface of these specimens. It is also comparatively 

 thinner than in T. cranium. Of a grey gelatinous aspect, it 

 is thickest on the lemon-yellow fleshy portions ; thinner, and 

 apparently not so solid, on the portions of surface from which 

 the spicula begin to project. It lias disappeared entirely 

 where the spicula are most numerous and projecting. The 

 canals of the intermediate or radiating masses become much 

 smaller and more numerous as they pass through the rind, on 

 the surface of which they open in the dark spots. 



Particular Description. 



The Skeleton. — The skeleton of this sponge, like those of 



