LANUGONYCH I A FLABELLIJM. 
105 
met, the diactines being particularly abundant. Judging by analogy I should 
say that the surface, the skeleton of which consists chiefly of true hexactines, is 
gastral, the other dermal. 
The hexactine and pentactine forms are orientated in such manner that 
four of their rays extend paratangentially whilst one protrudes vertically out- 
ward. The stauractines, triactines, diactines, and monactines are usually 
extended wholly paratangentially. 
The rhabds are 4-20 mm. long and 5-140 m thick near the middle. Those 
5-50 n thick are usually 4-7 mm. long. The slender ones are always distinctly 
centrotyle, the tyle being 1-6 ^ thicker than the adjacent parts of the spicule. 
In the stout rhabds the central tyle is only slightly developed, inconspicuous, 
and often altogether absent. The axial cross is equally developed in the stout 
non-centrotyle and the slender centrotyle rhabds. The smallest rhabds are 
nearly cylindrical and rounded at the ends. The rhabds 20-40 m thick in the 
middle taper gradually to 5-18 m towards the ends, which are usually unequally 
stout and simply rounded off. The large, stout rhabds generally have blunt, 
somewhat irregular, conic termini and are, just below the end, considerably 
thinner than the small slender rhabds. The measurements of five rhabds, 
tabulated below, indicate that these spicules are the more centrotyle and the 
more cylindrical the smaller they are, and vice versa. 
RHABDS. 
Length 
mm. 
Tyle 
Tliickness 
transverse 
diameter n 
difference be- 
tween transverse 
diameter of tyle 
and thickness 
of adjacent parts 
of the spicule 
of the spicule 
close to the tyle, 
near the middle 
of oue end fj. 
of the other 
end £i 
5 
19 
4 
15 
15 
12 
5 
23 
3 
20 
11 
5 
5.7 
56 
3 
53 
12 
8 
6.3 
53 
1 
52 
18 
14 
19 
80 
0 (no tyle) 
80 
5 
4 
The ends of the rhabds are, for a short distance, covered with small spines. 
Apart from this these spicules are smooth. The spiculation of the end-parts is 
more conspicuous in the small than in the large rhabds. 
