164 HYALONEMA (HYALONEMA) OBTUSUM. 



have rays which are in both varieties 4-6 m thick and covered with stout, blunt 

 (Plate 39, fig. 1) or pointed (Plate 39, figs. 11, 12), usually curved, oblique spines 

 2-4 M long. 



The slender acanthophores (Plate 39, figs. 2-4, 13-16, 22-24) are mostly 

 tetractine, triactine or diactine tetractine-derivates ; a few hexactine and pentac- 

 tine forms appear to be pinule-derivates. In both varieties the rays of these 

 spicules are sometimes 200 yu long and, at the base, in var. gracilis 1.3-1.5 n, in 

 var. robusta 1.5-3.5 m thick. They are usually curved more or less in an irregular 

 manner and bear sparse, irregularly distributed spines. In both varieties these 

 spines reach 6 /x in length and are usually more or less curved. The spines 

 arising from the end-parts of the rays are usually directed backwards and re- 

 curved; the others are either also recurved, or vertical, or directed outwards. 

 The basal parts of the rays appear always to retain their original, regular, relative 

 position. In the tetractines these parts of the rays form regular rectangular 

 crosses, in the triactines a T, and in the diactines usually a right angle. When a 

 ray entirely disappears, a large spine usually takes its place (Plate 39, fig. 16). 



The rhabds of the choanosome and gastral cone are for the most part blunt 

 amphioxes or amphistrongyles, but a few styles and tylostyles are also found 

 among them. 



The hlunt amphioxes and amphistrongyles of the choanosome and cone (Plate 

 33, figs. 1, 2, 18, 19, 21-23; Plate 39, fig. 39) are in both varieties nearly straight, 

 slightly curved, or angularly bent, and usually provided with a more or less 

 prominent central tyle. They are perfectly smooth. Their end-parts are 

 generally somewhat wavy in outline. The amphioxes (amphistrongyles) of the 

 upper part of the choanosome and the wall of the gastral cavity are 0.5-1.55 mm. 

 long and 4-20 m thick near the central tyle. The tyle is 2-8 m thicker than the 

 adjacent parts of the spicule, and measures 11-28 ^ in transverse diameter. 

 When there is an angular bend the apex of the angle invariably lies in the central 

 tyle (Plate 33, fig. 2). In the basal and the axial parts of the sponge these 

 amphioxes (amphistrongyles) attain a larger size. They are here 1-3.5 mm. and 

 more long and 8-50 m thick. 



The styles and tylostyles of the choanosome and gastral cone (Plate 33, fig. 20) 

 are in both varieties shorter than the isoactine rhabds above described, usually 

 only 0.9-1.6 mm. long. The largest terminal tyles of the tylostyles observed 

 were 30 m in diameter. 



The uncinate amphioxes of the superficial parts of the sponge (Plate 33, figs. 3, 

 4; Plate 39, figs. 25-30) are straight, or slightly curved, and sharp-pointed at 

 both ends. They are in var. gracilis 580 m long, and 4.5-9 n thick in the middle. 



