HYALONEMA (HYALONEMA) POLYCAULUM. 205 



at right angles, 3-8 n thick at the proximal end, conic, pointed, and covered with 

 somewhat sparse spines quite or nearly down to the base. The largest spines 

 are 5-15 m long and 2-3 n thick at the base. The basal spines arise vertically; 

 the distal ones are inclined towards the tip of the ray. 



The microhexactines (Plate 53, figs. 9-12, 14-16) measure 80-142 n in diam- 

 eter, and have fairly equal, straight, or onlj^ very slightly curved, conic, and 

 pointed rays, which -are joined at right angles in the centre of the spicule. The 

 rays are 2.5-4 n tliick at the base and covered along their whole length with 

 spines sometimes 0.4 fi long. The basal spines are sparse and vertical, the distal 

 more crowded and oblique, incUned backwards, towards the centre of the spicule. 



The rare microhexactine-derivates appear as spined aniphioxes, from the 

 centres of which arise terminally rounded rudiments of tlie four reduced rays. 

 In respect to size and spinulation they agree with the largest regular micro- 

 hexactines. 



The cfioanosomal rhabds are mostly centrotyle amphioxes, more rareh' 

 styles (tylostyles). They are mostly 1-2 mm. long and 10-20 ju thick. The 

 central tyle is relati^•ely much larger in the thin than in the stout amphioxes, and 

 measures 15-35 ju in trans\'erse diameter. The proportion of the thickness of 

 the parts of the spicule adjacent to the tyle and of the tyle itself is 100:110- 

 100:250. It is to be noted that the tjde, particularly in the slender rhabd, is 

 often very eccentric, the four rays, the remnants of which it represents, being 

 not all reduced to the same extent. 



The diactine (diactine-derivate) rhabd acanthophores of the hard superficial 

 knobs (Plate 53, fig. 17; Plate 54, figs. 16-20) are simple or centrotyle cylindrical 

 rods, often thickened, and usually densely spined, rarely smooth at the rounded 

 or spindle-shaped ends. They are straight or irregularity, sometimes (Plate 53, 

 fig. 17) very strongly curved. These spicules are usually 0.6-1.4 mm. long and 

 4-20 fi thick. The central tyle has a maximum in trans^'erse diameter of 35 //• 

 The smooth, strongly curved form (Plate 53, fig. 17) is onlj^ 5.5 /z thick, and 

 about 180 ju long measured along the curve. 



The upper ends of the stalk-spicules found in the parts of the body underlining 

 the hard superficial knobs attain 15 ^ in length, and are 50-110 m thick at the 

 lower, broken ends and attenuated above. In placesihese rods are irregular and 

 knotty. Their axial threads at these points exhibit remarkable irregularities, 

 from which I inferred that the spicules had here Ijeen broken and then again 

 joined by freshly apposed silica-layers. 



Of amphidiscs three kinds (which correspond to the large macrampliidiscs 



