102 



GEODIA MESOTRIAENA. 



Most of the cladomes of the anatriaenes He in a zone, about 1 mm. thick, just 

 below the lower limit of the sterraster-armour. The cladomes of most of the 

 adult ortho- and plagio-triaenes are situated at the lower limit of the sterraster- 

 armour, appearing as if they supported this layer of the cortex. Lower down 

 only young ortho- and plagio-triaene-cladomes are found. These are remarkably 

 few in number. In the middle and outer layers of the cortex no teloclade- or 

 mesoclade-cladomes occur. The rhabdomes of the protruding mesotriaenes 

 penetrate the whole cortex; their cladomes lie high above the surface. The 

 free, distal parts of these mesoprotriaenes compose the fur of the sponge. 



Besides these large spicules numerous small spicules, small rhabds, various 

 euaster forms, sterrasters, and, occasionally, sterroids are met with. 



The small rhabds are irregularly scattered in the interior of the choanosome 

 and form tuft-like groups in the outer layer of the cortex (Plate 23, fig. 25i; 

 Plate 25, fig. lb). Those in the proximal part of the choanosome are on the 

 whole similar to but smaller than those in the superficial tufts. It is therefore 

 to be supposed that these spicules are formed in the depth of the choanosome 

 and then pushed up to the surface. The fact, however, that hardly any such 

 spicules occur in the distal part of the choanosome and the lower and middle 

 layers of the cortex, which they would have to pass through on their way from the 

 interior of the choanosome to the superficial tufts, makes it somewhat doubtful 

 whether this supposition is correct. These small rhabds are anisoactine, the 

 thinner, more pointed end of those forming the superficial tufts being directed 

 outward. The tufts of these spicules in the outer cortical layer appear as coni- 

 cal groups, the apices of which are situated at the limit between the outer and 

 middle cortical layers. From these apices the spicules of each group (tuft) 

 radiate outwardly, penetrate the whole of the outer layer of the cortex, and ex- 

 tend, as stated above, some distance beyond it, so that their distal ends protrude 

 freely over the surface. These spicule-tufts lie quite close together, the neigh- 

 bouring ones coming distally nearly or quite in contact with each other. 



The sterrasters, between which occasionally a few sterroids occur, form a 

 rather dense mass in the middle layer of the cortex (Plate 22, figs. 3-5, 7; 

 Plate 23, figs. 24b, 25b; Plate 25, fig. le). Here only adult sterrasters occur; 

 young stages of these spicules are found in the lower cortical layer and the distal 

 part of the choanosome. In all the three specimens examined such young 

 sterrasters are, however, remarkably rare, which shows that, at the time of 

 capture, these spicules were in none of them being produced at all rapidly. 



The sponge is very rich in euasterforms. On the surface, both the outer 



