HYALONEMA (IIYALONEMA) POLYCAULUM. 
205 
at right angles, 3-8 n thick at the proximal end, conic, pointed, and covered with 
somewhat sparse spines cpiite or nearly down to the base. The largest spines 
are 5-15 n 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 /x in diam- 
eter, and have fairly equal, straight, or only 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 m thick at the base and covered along their whole length with 
spines sometimes 0.4 n long. The basal spines are sparse and vertical, the distal 
more crowded and oblique, inclined backwards, towards the centre of the spicule. 
The rare microhexactine-derivates appear as spined amphioxes, from the 
centres of which arise terminally rounded rudiments of the four reduced rays. 
In respect to size and spinulation they agree with the largest regular micro- 
hexactines. 
The choanosomal rhabds are mostly centrotyle amphioxes, more rarely 
styles (tylostyles) . They are mostly 1-2 mm. long and 10-20 m thick. The 
central tyle is relatively much larger in the thin than in the stout amphioxes, and 
measures 15-35 m in transverse 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 tyle, 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 irregularly, sometimes (Plate 53 , 
fig. 17) very strongly curved. These spicules are usually 0.6-1. 4 mm. long and 
4-20 n thick. The central tyle has a maximum in transverse diameter of 35 /x. 
The smooth, strongly curved form (Plate 53 , fig. 17) is only 5.5 m thick, and 
about 180 m long measured along the curve. 
The upper ends of the stalk- spicules found in the parts of the body underlying 
the hard superficial knobs attain 15 ^ in length, and are 50-110 n thick at the 
lower, broken ends and attenuated above. In places these rods are irregular and 
knotty. Their axial threads at these points exhibit remarkable irregularities, 
from which I inferred that the spicules had here been broken and then again 
joined by freshly apposed silica-layers. 
Of amphidiscs three kinds (which correspond to the large macramphidiscs 
