HYALONEMA (PRIONEMA) SPINOSUM. 275 



0.5-2.8 mm. long, and 20-100 m thick at the base; the lateral rays are straight, 

 and 0.2-1 mm. long. The length of the lateral rays is not in proportion to the 

 length of the apical ray, pentactines with a very long apical ray often having 

 comparatively short lateral rays and vice versa. 



The hexactine megasderes (Plate 49, fig. 19) have smooth, generally fairly 

 straight, conic rays. The intact ones measured have a maximum diameter of 

 0.5-1.5 mm. and rays 12-43 ^ thick at the base. Some fragments with rays 

 43 Ai thick, which I observed, indicated that these spicules occasionally attain 

 considerably larger dimensions. 



The acanthophores (Plate 48, figs. 1-10) have one to four straight or, more 

 rarely, curved rays, smooth in their basal part, but covered with spines in their 

 end-part. The rays are conic or, more rarely, cylindrical and blunt-pointed or 

 rounded, and not infrequently thickened at the end. These spicules measure 

 110-430 M ill maximum diameter (length); their rays are 10-30 ^ thick at the 

 base. The rays of the tetractine and triactine forms lie in one plane. In the 

 tetractine forms, which are the most frequent, either all four rays are fairly equal 

 (Plate 48, figs. 1, 3) or only three, the fourth being more or less reduced in length 

 (Plate 48, figs. 2, 4, 5) . In some of these spicules one ray is altogether suppressed. 

 These are the triactines (Plate 48, fig. 6). The diactines usually appear as 

 straight centrotyle rods (Plate 48, figs. 7-9). The rare monactines are tylostyles 

 (Plate 48, fig. 10). Their terminal tyle is obviously homologous to the central 

 tyle of the diactines. 



The rhabds. The rhabds of the gastral membrane are more or less curved, 

 slightly centrotyle amphioxes, and mostly about 1 mm. long and 8-9 n thick. 

 The central tyle measures 10-13 n in transverse diameter. In the interior simi- 

 lar and also much larger rhabds are met. The largest intact ones observed 

 measured 2.9 mm. by 40 m; the stoutest rhabd-fragment was 130 ^ thick. 



The six rays of the microhexadines (Plate 49, figs. 14-18, 20-23) are, in 

 the same spicule, fairly equal and regularly arranged, each one enclosing an 

 angle of 90° with its neighbours. The microhexactines measure 108-180 y. 

 in total diameter, usually 117-148 m- Their rays are straight, 50-90 m long, at 

 the base 4.5-6 ix thick, usually about 5 m, conic and sharp-pointed. They bear 

 large and conspicuous, backwardly directed spines. On the proximal half of 

 the ray the spines are 1.2-2.6 ix long, stout, and sparse; on its distal half they are 

 smaller and much more crowded. 



Among the amphidiscs four kind are to be distinguished: — large macram- 

 phidiscs, small macramphidiscs, large micramphidiscs, small micramphidiscs. 



