HYALONEMA (OONEMA) HENSHAWL 
329 
are similar to those of the gastrals and 42-70 n long. The proximal ray of 
the single hexactine form observed is 30 p. long. 
The canalar pinules (Plate 97 , fig. 6) are pentactine or, more rarely, hexac- 
tine. The distal ray is 120-220 /x long and 6-10 n thick at the base. The 
lateral rays are 53-110 n long; the proximal ray is, when present, 28-65 m long. 
All the rays are pointed, conical, and spined. The spines are very small, so that, 
even with the spines, the distal ray is nowhere thicker than at its base. 
The hypodermal and hypogastral pentactines. A large number of hypodermal 
pentactines were observed, but few hypogastral. The hypogastrals and hypo- 
dermals appear to be quite similar. Their rays are straight, smooth, conical, 
and blunt. The proximal ray is 550-900 n long and 26-47 n thick at the base. 
The lateral rays are 320-650 n long. 
The hexactine megascleres are 0.6-1 .4 mm. in diameter, and have fairly 
straight, conical, and blunt rays, 13-32 n thick at the base. 
The amphioxes are centrotyle, nearly straight or curved, rarely angularly 
bent near one end. They are 0.9-1. 6 mm. long and 8-23 /x thick near the 
middle. The central tyle is 12-27 n in transverse diameter, that is 1-6 /x more 
than the adjacent parts of the spicule. 
The rays of the microhexactines (Plate 97 , figs. 33-36) are nearly always 
perfectly straight; only quite exceptionally one of the rays exhibits a slight 
curvature. The microhexactines are 108-230 /x in diameter, generally 1 10 
190 m, and their conical, pointed rays are 3.5-7 /x thick at the base. The rays 
bear spines, the largest of which are 0.7-1. 5 n long. Generally the spines are 
sparsely scattered over the greater part of the length of the ray, leaving the 
distal end-part free for a distance of about 10 m- The proximal spines are 
vertical, the distal inclined backwards. 
Among the amphidiscs two kinds can be clearly distinguished morphologi- 
cally: — A, a stout kind with large anchors, about half the length of the whole 
spicule; and B, a slender kind with small anchors, much less than half, usually 
about a third, of the whole spicule in length. 
The length frequency-curve (Figure 20) has three main elevations sepa- 
rated by deep depressions. The part of the curve to the right of 106.72, the 
summit of which lies at about 179, pertains to the morphological group A, 
and comprises all amphidiscs of this kind. The deep depression (down to 0) 
between this part of the curve and the other parts shows that the amphidiscs 
it pertains to form a distinct group. This coincides with their morphological 
character, and so a special group must doubtlessly be established for these 
