416 STKUCTURE AND ECONOMY OF TETIIEA. 



2d, That in T. cranium the water and debris find an exit 

 through the more ancient portions of the rind ; 3d, That in 

 the Spitzbergen species the water and debris pass off through 

 the rents in the more recent portions of the surface — through 

 the open parts of the surface where the rind is thin or 

 deficient, but principally through the perforated grooves, 

 which are in fact partially repaired fissures. 



The pores of the Spitzbergen Tethea are the orifices of 

 tubes which pass through the rind. 



From what has been already stated, it will be perceived 

 that these tubes are arranged in groups of three ; the three 

 tubes of each group being set around the shaft of the corre- 

 sponding six-radiate spiculum, which is in fact the axis or 

 skeleton of the group. 



It must be recollected, however, that the branched heads 

 of the six-radiate spicula are arranged at various depths in the 

 rind ; and as they always appear to embrace a water-tube in 

 each of their primary angles, and are, besides, not placed per- 

 pendicularly under one another, so the tubes which proceed 

 inwards from the primary angles of the superficial to the 

 primary angles of the deeper spicula must be twisted or bent. 



Whether the water-tubes of the rind ramify I have not 

 loeen able hitherto to determine, but it appears to me probable 

 that they do, because the six-radiate spicula are more numer- 

 ous towards the surface of the rind than at its central aspect, 

 and therefore the groups of water-tubes must be proportionally 

 increased in number towards the periphery. 



I have already stated that it appears to me highly pro- 

 bable that in the younger, and therefore more symmetrical 

 form of this sponge, the six-radiate spicula of its rind are 

 arranged in systems so as to include a dodecahedron in six 

 octagons, that each octagon encloses a pair of pores, while the 

 dodecahedron includes in its own share the terminal bundles 

 of the intermediate portion of the skeleton, and is at the same 



