GEODIA OVIS. 



163 



In the inner layer of the cortex, below the sterraster-armour, large thick- 

 rayed oxyasters and also smaller euasters occur. The sterraster-armour layer 

 is occupied by sterrasters lying rather loosely and on an average four deep. It is 

 traversed by large megascleres the proximal parts of which take part in the 

 formation of the radial spicule-bundles above referred to, and the distal parts 

 of which protrude freely beyond the surface. The proximal parts of the minute 

 rhabds forming the dermal tufts are implanted in the sterraster-armour layer. 

 Many of these spicules traverse nearly the entire thickness of this layer and 

 extend down to within a short distance of its lower limit; others quite reach 

 that level, and some even protrude beyond it. The dermal layer is traversed 

 by tuft-like groups of more or less radially disposed minute rhabds which 

 diverge above (Plate 42, fig. 9). The proximal parts of these spicule-groups 

 are, as stated above, firmly implanted in the sterraster-armour layer; their 

 distal parts protrude freely beyond the surface for a distance of 100-300 [t, 

 occasionally as much as 400 /«, and form a sort of undergrowth at the base of the 

 spicule-fur. In the lower parts of the dermal layer large thick-rayed oxyasters, 

 similar to the subcortical ones, are met with. Its superficial part, that is, the 

 dermal membrane, is occupied by dense masses of small asters, for the greater 

 part strongylosphaer asters. 



The spicule-fur (Plate 40, fig. 5a; Plate 42, fig. 2), which, as stated above, 

 in places attains a height of 20 mm., is composed chiefly of large amphioxes, 

 mesoprocladcs, and anaclades. Tylostyles, proclade mesoproclade-derivates, 

 and orthotriaenes also occur in it. Most of these spicules are implanted in the 

 sponge with their proximal end, some appear to lie in it quite freely. The large 

 amphioxes of the spicule-fur (Plate 42, fig. 2b) are fairly uniform in size and 

 for the most part arranged radially. The mesoprocladcs and their proclade- 

 derivates are more variable, particularly in regard to the shape of the cladomc, 

 but all of the same order of magnitude and mostly arranged in a fairly regular 

 radial manner. The anaclades, which are nearly all anatriaenes, on the other 

 hand, exhibit an extraordinary diversity in size. Besides large, radially disposed 

 ones, numerous smaller and smallest anaclades are observed in the proximal jiart 

 of the spicule-fur, particularly on the lower surface of the sponge. The rhabdonies 

 of these small spicules are much and irregularly curved and although vertical 

 to the surface of the sponge in their basal part, do not extend radially through- 

 out their whole length. In places, particularly on the lower side, orthotriaenes 

 (Plate 42, fig. 2d) similar to the subcortical ones take part in the formation of 

 the proximal part of the spicule-fur. 



