406 STRUCTURE AND ECONOMY OF TETHEA. 



enclose polygonal spaces, generally hexagonal when complete 

 and well marked. 



On the dark-grey portions of the surface, where the pro- 

 jecting spicula are very numerous, the character of the surface 

 itself cannot be recognised ; on the lemon-yellow fleshy portions, 

 from which the naked spicula do not project, numerous darkish 

 spots are visible, of a polygonal form, and separated by 

 light yellow bands arranged in a sort of network. These 

 spots, under a lens of 1 or 2 inches' focus, appear to be per- 

 forated, and have a general resemblance to the pores of the 

 afferent orifices of the denser sorts of sponges. 



On various parts of the surface, but more abundantly 

 towards the attached pole of the mass, there are a variable 

 number of serpentine semi-cylindrical grooves, elongated, 

 ramified, and continuous, or short and disconnected. ' The 

 floor and walls of these grooves consist of a membrane per- 

 forated throughout by angular or rounded orifices, so as to 

 resemble an irregular network. The grooves may be here 

 and there observed to pass into deep fissures or vents in the 

 substance of the organism. These fissures tend towards the 

 centre in the direction of its long spicula, the bundles of 

 which may be observed passing into the interior on the walls 

 of the fissures. At the junction of the grooves and fissures 

 the perforated membranes of the former terminate in bands 

 and lamella?, suggesting the idea that the grooves are former 

 fissures closed in by the growth of the perforated membrane. 

 At these junctions also it may be observed that the recent 

 fissures are actually continuous with spacious cavities under 

 the perforated membrane, resembling fissures with their 

 deeper angular portions filled up and rounded off by new 

 sponge-texture, and traversed in their wider parts by bands 

 and lamellae attached to their walls and the deep surface of 

 the perforated membrane for the support of the latter. 



The perforated grooves are invariably situated on parts of 



