STRUCTURE AND ECONOMY OF TETHEA. 417 



time the typical position of the oscule or feculent orifice of the 

 system of sponge-groups by which it is surrounded. It there- 

 fore becomes a question, since the superficial spicula of the 

 rind are not arranged perpendicularly above the deeper, what 

 are their relations in this respect. I have not succeeded in 

 determining any relation ; but from what may be observed in 

 the more symmetrically developed T. cranium, I am inclined 

 to suspect that the successive strata of systems of spicula are 

 so arranged that lines passing through their centres would be 

 spiral. 



The comparatively minute canals of the rind become 

 larger as they pass into the intermediate or radiating masses 

 of the sponge. They no longer retain the same fixed relations 

 to the skeleton, but retiring from the immediate vicinity of 

 the spicular fasciculi they pass inwards, frequently anastomos- 

 ing in the interior of the intermediate spongy mass. The 

 anastomoses become more frequent as the tubes approach the 

 nucleus, in which they degenerate into areolae. 



The afferent water-canals are at first not to be distin- 

 guished from the more or less frequently anastomosing efferent 

 tubes ; but as they collect together into larger branches, and 

 converge outwards and downwards to the equatorial region, 

 and the attached polar region of the sponge, they become so 

 much larger, and their aspect is so peculiar, that they at once 

 catch the eye. Their walls are surrounded by, or rather 

 consist of, a semi-transparent matter, consisting of the peculiar 

 granular sarcode, intermixed with fibre-cells and stellate 

 spicula already described. These efferent tubes all terminate 

 at right angles or obliquely, mediately or immediately in the 

 fissures or perforated grooves on the surface, and the abundant 

 • ■■elutiuous material of their walls is continuous with the 

 matter of the same kind which I have already described as 

 filling nji and repairing the recent fissures and reducing them 

 to their perforated grooved form. The fissures result from the 



