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 fi long 

 and 3.6-5 n thick near the centre. The central tyle measures 5.8-15 n 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 m 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 rhabds 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 ix thick, rarely 95 n. 

 Most of them are blunt amphioxes or amphistrongyles, but amphityles, styles, 

 and tylostyles 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 m thick; their spherical terminal tyles 

 measure 40-50 ^ iii diameter. A short somewhat spindle-shaped style 75 m 

 thick at the rounded end and 95 m 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 M or more in transverse diameter. 



The modified basal acanthophore 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 ju thick near the centre. The central 

 tyle is « 12.5-27/:* in transverse diameter, the terminal thickenings 12-30 m- 

 The spherical terminal thickenings are stouter than the spindle-shaped. The 

 spiny end-parts are 80-260 m 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. 



