HYALONEMA (OONEMA) B I AN CHORATUM PINULINA. 
319 
These spicules are similar to the corresponding basal spicules of specimen a, 
and differ from these only by being thicker and more spiny. While in the 
majority of the acanthophores in the sponge the proximal and middle-parts of 
the rays are smooth and only the distal part spiny, we find among the (similar) 
spicules of the Palythoa skeleton relatively many fewer with rays smooth in their 
proximal part, the majority being here entirely spined. it is to be noted also 
that the spicules of the Palythoa skeleton have, on the whole, larger spines than 
the corresponding spicules in the basal part of the sponge. In the larger spicules 
of the Palythoa skeleton the spines are on the distal parts of the rays very much 
larger than on their proximal parts. In the smaller forms this difference is not 
nearly so conspicuous, and in the smallest all the spines appear to be fairly equal 
in size. The largest spines measured were 15 /x long, and 14 n broad at the base. 
The average thickness of the rays 
of the rhabd acanthophores of specimen b is 16.0 n 
“ “ “ “ “ “ a “ 2.5.4 /d 
“ “ “ “ “ the Palythoa on the stalk of specimen .. a “ 32.4 /u 
“ “ tri- to tetractine acanthophores of specimen b“ 22.4^ 
“ “ “ “ “ “ “ “ a “ 31.0 m 
“ “ “ “ “ “ “ the Palythoa on the stalk of 
specimen a “ 31.6 ju 
Thus these spicules are in a much stouter than in b, and in the Palythoa 
attached to the stalk of a thicker than in the sponge itself. 
The fact that the Palythoa spicules are, on the whole, stouter and more 
spiny than those of the sponge is, no doubt, due to the Palythoa selecting for the 
purpose of building its skeleton the stoutest and most spiny of the spicules shed 
by the sponge. That these spicules are in a (on the stalk of which Palythoa 
polyps with sponge-spicule skeletons grew) much stouter than in b (the symbio- 
tic polyps of which have no skeleton) either may have nothing to do with their 
symbiotic polyps, and be in respect to them accidental; or it may be due to 
an influence of the spicule-requiring Palythoa on the sponge, comparable to 
that of a gall-wasp clutch on the vegetable tissue surrounding it; an influence 
which, in this case, might cause the sponge to produce abnormally stout and 
spiny acanthophores. 
The microhexactines (Plate 82, figs. 2-11, 20), which are the same in both 
specimens, have equal, regularly arranged rays and measure 57-152 ^ in total 
diameter. The rays are 2-3.8 n thick at the base, conical, and attenuated distally 
to a fine point. They are straight in their proximal part and usually curved 
