HYALONEMA (PRIONEMA) AGUJANUM. 257 



two specimens of var. tenuis, form A, bear Palythoa on their stalk, I shall only 

 describe these spicules of this form. 



The entirely spined short-rayed acanthophores (Plate 76, figs. 4-6, 17-30, 34) 

 of var. tenuis, form A, are mon- to tetractine. The rays of the tri- and tetrac- 

 tines always lie in the same plane. The rays are cylindrical and rounded, 

 sometimes also thickened at the end. The whole spicule is quite uniformly and 

 densely covered with spines, which arise vertically or more rarely obliquely 

 from its surface. The obhque spines, which invariably point outwards, are 

 confined to the ends of the rays. The spines are straight, conical, 4-6 yu long, 

 and 5-7 fi broad at the base. The entirely spined basal spicules measure 58- 

 210 M in maximum diameter (length), and their rays are 14^5 n thick. It is to 

 be noted that this thickness is by no means always in proportion to the length of 

 the spicule. In the smaller forms, under 85 m in length, the rays are 15-22 /j. 

 tliick; in the intermediate, 85-100 ^ in length, they are 14^5 ^ thick, and in 

 the larger, over 100 ^ in length, they are 15-34 yu thick. 



In view of the fact that the Palythoa doubtlessly derives the whole of the 

 material wherewith it builds its skeleton from the basal part of the sponge to the 

 stalk of which it is attached, it appears very remarkable that the basally smooth 

 spicules, so frequent in the lower part of the sponge, are relatively so rare in the 

 Palythoa; and that, vice versa, the entirely spined spicules, forming the bulk of 

 the skeleton of the Palythoa, are so rare in the lower part of the sponge. In 

 Hyalonema (Hyalonema) grandancora, where the relation between the skeleton 

 of the lower part of the sponge-body and the skeleton of the Palythoa is the same, 

 this difference appears to be due to the Palythoa selecting the stoutest and most 

 spiny acanthophores of the sponge as material for building its skeleton. 



The stalk-spicules (Plate 75, figs. 29, 30; Plate 78, figs. 1, 2), at the point 

 where they arise from the body, are in var. tenuis 180-600 /x thick, in var. lata 

 60-300 M- All those of var. tenuis appear to be smooth. Only some of those of 

 var. lata are smooth, the others (Plate 78, figs. 1, 2) being provided with annular 

 constrictions, usually much deeper on one side than on the other. These con- 

 strictions have a maximum depth of 12 jx, and follow each other at fairly equal 

 intervals of about 110 fi. They render the outline of the parts of the spicules, 

 where they occur, wavy in appearance. In these regions the axial thread ex- 

 hibits a more or less clearly pronounced thickening at or near the centre of many 

 of the bulging parts lying between successive constrictions (Plate 78, figs. 1,2). 

 Some of the stalk-spicules of this variety are irregularly rounded at the proximal 

 end. In one of them the rounded proximal end is 220 n thick. 



