HYALONEMA (HYALONEMA) AGASSIZI. 
187 
more or less centrotyle spiny rods, pointed at both ends. Their middle-part is 
straight, their end-parts are slightly curved. These spicules are 84-240 n long 
and 3.6-5 m thick near the centre. The central tyle measures 5.8-15 /x in trans- 
verse diameter and, when large, clearly shows that it is composed of four ray-rudi- 
ments. 
The monactine microhexactine-derivates are very rare. One of form A 
which I measured was 75 /x long, and 3.5 ^ thick at the rounded end. 
Apart from the diactine pinules and the diactine and monactine derivates 
of the stout-rayed basal tetractines and the microhexactines above described, 
three kinds of rhabds can be distinguished : — ordinary rliabds of the choanosome 
and axial column, modified rhabds of the basal part of the sponge-body, and 
uncinates. 
The ordinary rhabds of the choanosome and axial column (Plate 45 , figs. 19- 
22) are smooth, slightly curved or, rarely, angularly bent, 0.3-7 mm. and more 
long, as some long fragments observed indicate, and 7-50 n thick, rarely 95 /x. 
Most of them are blunt amphioxes or amphistrongyles, but amphityles, styles, 
and t.ylostyles also occur among them. The smaller amphioxes, amphistrongyles, 
and amphityles are generally distinctly centrotyle. Remarkably regular cylin- 
drical amphityles occur in the marginal part of the gastral wall of form A. These 
are mostly 0.7-1 mm. long and 26-30 n thick; their spherical terminal tyles 
measure 40-50 ^ in diameter. A short somewhat spindle-shaped style 75 /x 
thick at the rounded end and 95 /x at the stoutest point was observed in the axial 
column of this form. The axial canals (threads) of the small rhabds are usually 
quite fine, those of the large ones on the other hand generally wide, sometimes 
5 /x or more in transverse diameter. 
The modified basal aca,nthopliore rhabds (Plate 45 , fig. 5) are centrotyle, usu- 
ally slightly curved, smooth in the middle, and strongly spined at the ends, which 
are generally somewhat thickened. The terminal thickenings are either spheri- 
cal or spindle-shaped, and in the same may be either similar or dissimilar, one 
end-thickening frequently being spherical, the other spindle-shaped. These 
spicules are 0.8-2. 6 mm. long, and 7-22 ^ thick near the centre. The central 
tyle is 12.5-27 m in transverse diameter, the terminal thickenings 12-30 /x. 
The spherical terminal thickenings are stouter than the spindle-shaped. The 
spiny end-parts are 80-260 n long. The axial canals (threads) of these spicules 
are often very wide. They are usually closed in the rounded ends and open in 
the spindle-shaped. 
The uncinates are mostly diactine, but monactines also occur. 
