HYALONEMA (OONEMA) BI ANCHOR ATUM 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 m long, and 14 y. broad at the base. 

 The average thickness of the rays 



of the rhabd acanthophores of specimen 6 is 16.0 ^ 



" " " " " " a "25.4 m 



" " " " "the Palythoa on the stalk of specimen . . a " 32.4 ^ 



" ." tri- to tetractine acanthophores of specimen 6 " 22.4/* 



" " " " " " " " a "31.0m 



" " " " " " " the Palythoa on the stalk of 



specimen a " 31.Gm 

 Thus these spicules are in a much stouter than in h, 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 h (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 microhexadines (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 ix thick at the base, conical, and attenuated distally 

 to a fine point. They are straight in their proximal part and usually curved 



