HYALONEMA (PRIONEMA) AGUJANUM. 255 



middle-part, together with the spines, usually about 6 ^ thick. The rays to be 

 considered as the laterals are 60-120 m long. The proximal ray of the hexactine 

 forms is 70-115 ii long. The hexactine and the pentactine forms appear to be 

 fairly equally abundant. 



The (hyiiodermal and hypogastral) pentadines (Plate 72, fig. 19) have 

 smooth, conical, and straight, terminally rounded rays. The proximal ray is 

 460-900 n long and 17-34 n thick at the base. The lateral rays are 220-500 ix 

 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 M 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 n 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 aU 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 ^ thick near the middle, and generally 

 curved or, more rarely, angularly l^ent. 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 yu, thickness 

 of one end 12, of the other 19 ix. 



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 Palythoa). 



