212 
HYALONEMA (HYALONEMA) PLACUNA. 
one shorter and terminally rounded and usually thickened ray; and short and 
stout cent.rotyle amphioxes. 
The ordinary centrotyle amyliioxes (Plate 63 , fig. 46) are in both forms usu- 
ally 0.8-2 n long and 9-21 n thick near the centre. The central tyle is 14-22.5 n 
in diameter. The ends are blunt and usually irregular, with widened axial 
thread-ends. In these spicules the proportion of thickness (close to the tyle) 
to length is 1 : 50-72, on an average 1 : 63.3. 
The anisoactine centrotyle rhabds appear as centrotyle tylostyles or, more 
rarely, styles. They are shorter than the isoactines, and 11-23 n thick near the 
terminal tyle (rounded end). The terminal tyle is often irregular, and measures 
in transverse diameter 13.5-37 n, sometimes 14 n more than the adjacent part of 
the spicule. 
The short and stout centrotyle amphioxes (Plate 63 , fig. 47) are 0.6-1. 6 mm. 
long, and 13-38 /u thick near the middle. The central tyle is 23-45 u in diameter. 
The proportion of the thickness (close to the tyle) to the length is 1 : 37-52, on 
an average 1:44. 
The hypodermal pentactines (Plate 63 , figs. 48-50) appear to be about the 
same in both forms. It is to be noted, however, that forms with lateral rays over 
480 fi in length were found only in form A. The proximal ray is usually slightly 
curved, conic, blunt, 470-800 n long, and 9-55 u thick at the base. The lateral 
rays may be fairly equal, or unequal in size. Sometimes their inequality is very 
great (Plate 63 , fig. 48), the longest being nearly twice as long as the shortest. 
They are conic, very blunt, straight, or curved concave to the proximal ray, and 
usually also inclined in this direction. The longest lateral ray of the spicule is 
220 ju-1.1 mm. long. 
Hypogastral pentactines were found only in the preparations of form B, and 
here also they are very scarce. Those observed had lateral rays 360-670 m long 
and 18-35 n thick at the base. 
The hexactine megascleres are scarce, but have been found in both forms. 
In both they measure 0.8-1. 2 mm. in diameter and have blunt conic rays 16-34 u 
thick at the base. 
The microhexactines and microhexactine-derivates form a series beginning 
with spicules composed of six fairly equal rays and ending with centrotyle diac- 
t.ines from the central tyle of which arise one to four ray-rudiments. The most 
frequent are the intermediate forms, representing the middle-part of this series, 
with two opposite rays longer than the other four. 
The regular microhexactines in both forms (Plate 64 , figs. 4, 6, 7; Plate 65 , 
