HYALONEMA (HYALONEMA) PLACUNA. 
211 
The ordinary gastral pinules of form B (Plate 65 , figs. 16-18) are similar. 
All those observed were pentactine. Their distal ray is 164-286 m long, rarely 
330 n, 8-11.5 n thick at the base, and at the point of maximum thickness li- 
ly. 5 m thick. Everywhere, except at the base and at the tip, it bears spines, 
which are larger (17-25 n long) and, particularly the upper, more divergent than 
in form A. In many of these spicules the lower spines are irregular and branched 
(Plate 65 , fig. 17). The maximum thickness of the distal ray, together with the 
spines, is 28-52 /x. The lateral rays are conic, pointed or blunt, 40-73 pi long, 
and either quite smooth or provided with a few very minute spines. 
Besides the pinules described above, other much smaller pinules with appar- 
ently rudimentary lateral rays (Plate 65 , figs. 9-12) have been found in the spicule- 
preparations of the interior of form B. As I have not seen them in situ in the 
sections, I cannot say with certainty whether they are gastral or canalar. The 
probability is that they are canalar. These pinules are pentactine. Their 
distal ray is straight and 172-200 /x long, 7.5-9 pi thick at the base. It is some- 
what spindle-shaped and measures in thickness 11-13 pi at the point of maximum 
thickness, which is about a third of the length of the distal ray distant from the 
base. The distal ray ends with a terminal cone. Everywhere, except at its 
base and at its tip, it bears large and sparse, more or less irregularly arranged 
spines. The lower spines arise steeply or vertically from the ray and are often 
branched; the upper are inclined towards the tip and simple; the latter decrease 
in size distally. The lateral rays appear as short stumps only 10-14 pi long. 
Sometimes it seemed to me that their shortness was due to their being broken; 
in other cases they appeared to be quite intact. Occasionally one or a few 
large and slender spines arise from the lower side of the laterals. Sometimes a 
large spine of this kind projects downward from the centre of the spicule (Plate 65 , 
fig. 11). Such a spine appears as a rudiment of a sixth proximal ray. 
The diactine marginal pinules. In the somewhat fragmentary specimen 
of form B the margin is torn off and these spicules are missing. In form A they 
are abundant. In this form they are slightly curved or nearly straight, and 0.9- 
1.5 mm. long. The distal ray may be longer or shorter than the proximal. 
The former measures 520-700 fx in length, the latter 360-760 pi. At their base 
both rays are 9-1 1 pi thick. The centrum is thickened to a tyle 1 1-20 n in trans- 
verse diameter. The distal part of the distal ray bears spines strongly inclined 
towards the tip. This spiny part, which is usually 350-400 pi long, has, together 
with the spines, a maximum diameter of 26-30 pi. 
Of rliabds three kinds can be distinguished: — ordinary, isoactine, and centro- 
tyle amphioxes; anisoact.ine centrotyle rhabds with one longer and pointed and 
