188 HYALONEMA (HYALONEMA) AGASSIZI. 



The diaciine uncinates (Plate 44, figs. 6-14, 17a) are generally straight oi' 

 slightly curved, simple amphioxes; considerably curved and centrotyle ones, 

 however, also occur. The ordinary amphiox uncinates in form A are 330-800 ^ 

 long, 5.5-12 n thick in the middle, and beset with splines. As far as I could make 

 out these spines are 0.7-1.5 m long, and about 1 n thick at the base. Sometimes 

 it appeared as if they were continued in a fine terminal filament which was, 

 however, too thin to be distinctly projected even with the 280 n light. At one 

 end of the spicule these spines are numerous, rather close together, and strongly 

 inclined toward the opposite end. Toward the other end they become much 

 scarcer and less inclined. Some of the spines nearest the latter end are vertical 

 or even inclined in the opposite direction. In the centrotyle uncinates the cen- 

 tral tyle is 15% to 45% thicker than the adjacent parts of the spicule. 



The monactine uncinates appear as tylostyles. In form A they are 260- 

 293 fi long and 9-12 n thick just below the rounded end. The rounded end itself 

 is thickened to a more or less spherical tyle 14-16 n in transverse diameter. 



The large stalk-spicules of form A (Plate 41, fig. 2; Plate 43, figs. 1-7) have 

 a maximum length of 42 cm. and all are broken off at the lower distal end. Wliere 

 they arise from the sponge they are 0.05-0.95 mm. thick; 30 cm. lower, where 

 most of them are stoutest, they are 0.5-1.3 mm. thick. 



One (Plate 43, fig. 1), which I studied in detail, is 160 m thick at the upper 

 end, and rapidly increases in thickness to 730 m at 7 cm. from the end; it then 

 gradually thickens down to 28 cm., where it attains its maximum thickness 

 of 1050 n. Farther on it again becomes thinner, and at the lower end, 42 cm. 

 from the tip, is 760 ix thick. Its axial thread is for the most part thin. It is 

 thickened, however, here and there in an irregular manner. The silica is very 

 clearly stratified. The surface of the upper, proximal part of the spicule is quite 

 smooth. Where the spicule attains its maximum thickness fine transverse lines 

 (Plate 43, fig. 7) make their appearance on its surface, and 1 cm. above the distal 

 end its surface, for a short distance, has quite a peculiar structure (Plate 43, 

 fig. 4). Here a silica-layer is exposed which consists of lamellae overlapping 

 like tyles, and composed of parallel rods about 10 yu thick and lying close to- 

 gether. These rods extend nearly but not quite paratangentially and longitudi- 

 nally. They deviate slightly both radially and laterally from the direction of the 

 axis of the spicule. The radial deviation is due to their forming the overlapping 

 lamellae, and like the lamellae themselves they slightly diverge from the axis 

 below. The lateral deviation is due to their lying somewhat obliquely in elon- 

 gated spirals. 



