HYALONEMA (HYALONEMA) TENUIFUSUM. 225 



in length, rounded at the end, and thickened so as often to attain a transverse 

 diameter nearly equal to that of the "central" tyle, which in these spicules is of 

 course very eccentric. 



The spicules forming the stalk are, at the point where they arise from the 

 sponge-body, sometunes 0.5 mm. thick. Fragments of rhabds 20-40 /x thick 

 found in spicule-preparations of the interior are probably parts of young stalk- 

 spicules. All the stalk-spicules observed were smooth. 



The stout acanthophores (Plate 67, figs. 6, 7) have from one to six, most 

 frequently from two to fom* rays. The rays usually taper distally, more rarely 

 they are cyUndrical. The end-part is densely covered with spines, generally 

 somewhat thickened, and terminally rounded, or more rarelj^ pointed. The 

 diactine forms are centrotyle. The three- to six-rayed forms are 335-580 fi 

 in diameter, and have rays 10-20 ^ thick. In the three-rayed forms all the rays 

 he in the same plane; two generally in a straight line, and the third at right 

 angles to these. In the four-rayed forms the rays also lie in one plane, and the 

 adjacent ones enclose angles of 90°. These spicules therefore appear as crosses 

 (stauractines) . In the rare pentactine forms f om- rays extend in a plane, enclose 

 angles of 90° with each other and appear as lateral rays, whilst the fifth is verti- 

 cal to the plane of the four others and appears as an apical ray. The rare six- 

 rayed forms are regular hexactines. 



In the diactine acanthophores the rays lie in a straight or slightly curved 

 line. These spicules are 675 ii-l.l mm. long and 10-12 fj. thick. Their central 

 tyle is 17-30 ^ in transverse diameter. They are connected by transitional forms 

 with the ordinary centrotyle amphiox megascleres. These transitional forms 

 are about as thick as the true diactine acanthophores, but longer, reaching 2.6 

 mm. in length, and have smaller central tyles. The monactine forms are tylo- 

 styles 0.8-1 mm. long and 10-13 m thick. The terminal tyle is about 22 fi in 

 diameter. 



The regular microhexactines (Plate 67, figs. 10, 11) have six regularly dis- 

 tributed, conical and sharp- pointed, straight or slightly cui-ved rays. The 

 curvature is, when present, usually greater in the proximal than in the distal 

 part of the ray. The rays are nearly smooth, or slightly roughened by the 

 presence of exceedingly minute spines, which appear to be directed backwards 

 towards the centre of the spicule. In specimen (a) the regular microhexactines 

 are 110-155 jx in total diameter, and have rays 1.5-2 tx tliick at the base. In 

 specimen (6) these spicules are somewhat smaller, 80-105 n in diameter, and 

 have rays 1-2 /i thick at the base. 



