HYALONEMA (SKIANEMA) AEQUATORIALE. 
379 
“central” tyle is often situated a considerable distance away from the middle 
of the length of the spicule, many of these centrotyle amphioxes being markedly 
anisoactine. 
The styles and tylostyles are, like the amphioxes above described, centrotyle 
diactine rliabds. One of their rays is similar to an amphiox-ray, the other is 
reduced in length and rounded, and generally also thickened at the end. These 
spicules are 0.5-2 mm. long. They are slightly thickened at the morphological 
centre, in which the axial cross can always be made out, and are here 8-25 y 
thick. The rounded end (terminal tyle) is 8-40 y in diameter and usually 
separated from the remaining part of the spicule by an attenuation. In this 
attenuation, or neck, the spicule is 1-11 y thinner, usually 3-6 y, than the rounded 
end (terminal tyle). 
The large amphioxes of the rhabd-bundle which forms the skeletal axis of the 
sponge-body and terminates in the gastral cone are 2 mm. and more (the long 
ones are broken) long and 30-60 y thick. 
The spheres are regularly spherical, oval, or irregular, potato-shaped. 
They measure 17-170 y in maximum diameter, most frequently about 30 y. 
All contain a granular centrum round which silica-layers of somewhat varying 
refractory index have been deposited. The surface is in the smaller spheres 
regular, smooth, and continuous, in the larger it is usually irregular. As an 
example I shall describe a typical large sphere. This spicule is 168 y long and 
157 y broad. It has an oval granular centrum 12 y long and 7 y broad. The 
granules in it are numerous, and about 1 y in diameter. In the silica, which is 
perfectly hyaline, a concentric stratification around the centrum can be made 
out very clearly. In one place a watchglass-shaped granular body, which 
appears sickle-shaped in profile (optical section), is interpolated between two 
successive layers of ordinary hyaline silica. A number of groove-like inden- 
tures, sometimes 2 y deep, are visible on the surface of the sphere. Several of 
these radiate from one point. 
* 
The microhexactines (Plate 99 , figs. 3-10, 32-35) are 60-96 y in maximum 
diameter. In some all the rays are fairly equal; in others two opposite rays are 
considerably longer than the other four. The latter are sometimes nearly twice 
as long as broad. The rays are 1.8-2. 4 y thick at the base. They are conical, 
finely pointed, and covered with very minute spines. The basal part, usually 
about half of the total length of the whole ray, -is nearly straight, the distal part 
curved. This curvature is usually greater at the point where the basal straight 
part passes into the distal curved part than farther on. The whole curvature is 
