HYALONEMA (LEPTONEMA) CAMPANULA. 247 
The hexactine megascleres are 550 »-1.8 mm. in diameter. Their rays are 
7-22 u thick at the base, usually more or less curved, and only slightly attenuated 
toward the rounded end. The end-parts of the rays, particularly of the smaller 
hexactines, bear minute spines. 
The diactine megascleres are centrotyle amphioxes. They are usually 
1-1.5 mm. long and 9-18 » thick near the centre. The central tyle is 12-22 y 
in transverse diameter, that is 2-4 « more than the adjacent parts of the spicule. 
The basal pentactine and diactine acanthophores are similar to the ordinary 
pentactines and diactines of the body, above described, but have rays reduced 
in length and somewhat thickened and spined at the end. 
The stalk-spicules (Plate 81, fig. 11) have a maximum thickness of 110 u 
and all are broken off at the lower, distal end. Their proximal, upper parts are 
smooth. Farther down minute, strongly inclined, upwardly directed spines 
begin to make their appearance. Distally these spines become larger and appar- 
ently also less numerous. The spines are partly scattered, partly arranged in 
oblique (spiral) transverse rows. 
The microhexactines (Plate 81, figs. 3-6) are 50-100 » in diameter. The 
rays are 1-1.5 » thick at the base, straight in their proximal, but curved in their 
distal part. This curvature is either fairly uniform or considerably greater just 
beyond the point where it begins than in the end-part, and on the whole such 
that the tips of adjacent rays come to be parallel or convergent. 
Morphologically two kinds of amphidiscs can be distinguished: — those 
with relatively thin shaft and slender, somewhat bell-flower shaped anchors; 
and those with relatively stout shaft and broader, oval anchors. The former 
are 150 » or more long, whilst the largest of the latter is only 118 w long. As the 
adjoined graph shows, a rather conspicuous depression in their length frequency- 
curve divides the large and slender-anchored amphidises biometrically into a 
larger and a smaller kind. In view of the morphological identity of the larger 
and smaller, I think that all these slender-anchored amphidises can be considered 
as amphidises of the same kind, and I shall describe them as macramphidiscs. 
The broad-anchored amphidises, 118 » or less in length, are divided bio- 
metrically by a very wide gap in the length frequency-curve, situated between 
26 and 77 yu, into a larger and a smaller kind. The larger of these amphidiscs 
I shall describe as mesamphidises, the smaller as micramphidises. Since the 
length frequency-curve in both exhibits several depressions, neither of them 
can be said to form a biometrically homogeneous group. Since, however, 
those depressions are not very great and since these amphidises are so rare that 
