HYALONEMA (PRIONEMA) AGUJANUM. 
255 
middle-part, together with the spines, usually about 6 n thick. The rays to be 
considered as the laterals are 60-120 m long. The proximal ray of the hexactine 
forms is 70-115 n long. The hexactine and the pentactine forms appear to be 
fairly equally abundant. 
The (hypoclermal and hypogastral) pentadines (Plate 72 , fig. 19) have 
smooth, conical, and straight, terminally rounded rays. The proximal ray is 
460-900 m long and 17-34 n thick at the base. The lateral rays are 220-500 n 
long. In the same spicule they are usually very unequal in size, the largest 
being sometimes as much as twice as long as the smallest. 
The hexactine megascleres (Plate 72 , figs. 26, 27) are regular or, more rarely, 
two opposite rays are longer than the other four. They measure 0.6-1. 4 mm. 
in maximum diameter (length), and their straight, conical, blunt rays are 14- 
33 n thick at the base. 
Most of the rhabds of the body are centrotyle amphioxes, but tylostyles 
have also occasionally been observed. The centrotyle amphioxes are 0.8-3. 3 
mm. long and 8-19 m thick near the centre. The central tyle is 11-23 m in 
transverse diameter, that is 1.5-12 ^ more than the adjacent parts of the spicule. 
These spicules attain a larger size in var. lata than in var. tenuis. 
Among the basal acanthophores two kinds can be distinguished: — forms 
with long and slender rays, and forms with short and stout rays. The spicules 
of the first kind are all diactine, those of the second kind mon- to pentactine. 
The long and slender diactine acanthophores are connected by numerous 
transitional forms with the ordinary rhabds of the upper parts of the body. 
They are 0.6-1. 6 mm. long, usually 6-9 n thick near the middle, and generally 
curved or, more rarely, angularly bent. The two rays of the angular forms 
are usually fairly straight. The curvature or angular bend of these spicules 
is sometimes very considerable, the latter occasionally such that the angle en- 
closed by the two rays is nearly a right one. The spined end-parts of the rays 
are often more or less thickened and often unequal. The following dimensions 
of a spicule of var. lata may serve as an example of this kind of spicule unequally 
thickened at the two ends: — -length 1.4 mm., thickness in middle 9 tx, thickness 
of one end 12, of the other 19 /x- 
The stout and short mon- to pentactine acanthophores can again be divided 
into two groups of forms only slightly connected by transitions: — those with 
rays smooth in their basal part, spined only at the end, and longitudinally less 
reduced (found chiefly in the sponge) ; and forms with rays spined throughout 
their length and longitudinally more reduced (found chiefly in the Palyt.hoa). 
