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 rhabds. 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 m 

 thick. The rounded end (terminal tyle) is 8-40 ^ in diameter and usually 

 separated from the remaining part of the spicule by an attenuation. In this 

 attenuation, or neck, the spicule is 1-1 1 // thinner, usually 3-6 /x, 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 /u thick. 



The spheres are regularly spherical, o\'al, or irregular, potato-shaped. 

 They measure 17-170 ai in maximum diameter, most frequently about 30 n. 

 A\\ contain a granular centrum round which silica-layers of somewhat varjnng 

 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 /^ long and 

 157 /i broad. It has an oval granular centrum 12yu long and 7 y. broad. The 

 granules in it are numerous, and about 1 m 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 fi 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 ^ in maxinumi 

 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 ^u 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 



