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 /x long, rarely 

 330 ju, 8-11.5 M thick at the base, and at the point of maximum thickness 11- 

 17.5 n thick. Everywhere, except at the base and at the tip, it bears spines, 

 which are larger (17-25 ii 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 n. The lateral rays are conic, pointed or blunt, 40-73 ix long, 

 and either quite smooth or provided with a few very ininute 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 thespicule- 

 preparations of the interior of form B. As I have not seen them in situ in the 

 sections, I cannot say with certainty whether the,y are gastral or canalar. The 

 probability is that they are canalar. These pinules arc pentactine. Their 

 distal ray is straight and 172-200 n long, 7.5-9 m thick at the base. It is some- 

 what spindle-shaped and measures in thickness 11-13 n 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 n 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 cur\ed 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 n in length, the latter 360-760 ix. At their base 

 both rays are 9-11 ^ thick. The centrum is thickened to a tyle 11-20 ^ 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 m long, has, together 

 with the spines, a maximum diameter of 26-30 /x. 



Of rhabds three kinds can be distinguished: — ordinary, isoactine, and centro- 

 tyle amphioxes ; anisoactine centrotyle rhabds with one longer and pointed and 



