414 STRUCTURE AND ECONOMY OF TETHEA. 



the spiculum. As a result of this arrangement, it may be 

 observed that when two or more of the six-radiate spicula are 

 symmetrically arranged, each elongated octagonal mesh in- 

 cludes two pores — one at each of its extremities, and belong- 

 ing to distinct groups or systems. Dr. Johnston has denied 

 the existence of pores and oscula in Tdhca cranium. I have, 

 however, satisfied myself that pores of that sponge are situ- 

 ated in the furrows between the conical papilla?. Dr. 

 Johnston's observations were made on a specimen preserved 

 in spirits, with which I supplied him ; my own observations 

 were made on similar specimens. Neither Prof. Ed. Forbes 

 nor I could detect with the naked eye, or by means of lenses 

 of low power, either pores or oscula in the living sponge 

 examined in sea-water. But as the specimens had been for 

 some hours out of the water, and were not examined after a 

 sufficiently long interval of undisturbed repose, I am inclined 

 to attribute our failure in detecting the contractility of 

 Tethea, and also the pores and oscula, as had previously been 

 done by Audouin and Milne Edwards, to the continued con- 

 traction produced by the disturbance of the animal. 



I have not been able to detect in T. cranium any trace 

 of oscula ; but if an inference may be drawn from their 

 situation in other sponges, I would be inclined to look for 

 them on the conical spicular elevations of the rind. 



I have already referred the somewhat elevated spaces, 

 bounded by the rectangular extremities of the six-radiate 

 spicula, or the assumed corresponding dodecahedral spaces 

 of this new species, to the conical spicular elevations of T. 

 cranium ; I am therefore inclined to believe, that if oscula 

 exist at all in the form now under consideration, they must 

 be sought for in these spaces. 



It is interesting, in connection with this question, to 

 observe that the recent rents or fissures in the rind are 

 generally margined by the denuded quadrangular extremities 



