HYALONEMA (LEPTONEMA) CAMPANULA. 247 



The hexactine megascleres are 550 /i-1.3 mm. in diameter. Their rays are 

 7-22 M 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 diadine megascleres are centrotyle amphioxes. They are usually 

 1-1.5 mm. long and 9-18 m thick near the centre. The central tyle is 12-22 yu 

 in transverse diameter, that is 2-4 n more than the adjacent parts of the spicule. 



The basal pentactine and diadine 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 yu 

 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 /x in diameter. The 

 rays are 1-1.5 n 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 shai:)ed anchors; 

 and those with relatively stout shaft and broader, oval anchors. The former 

 are 150 m or more long, whilst the largest of the latter is only 118 ;u long. As the 

 adjoined graph shows, a rather conspicuous depression in their length frequency- 

 curve divides the large and slender-anchored amphidiscs 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 amphidiscs can be considered 

 as amphidiscs of the same kind, and I shall describe them as macramphidiscs. 



The broad-anchored amphidiscs, 118 m or less in length, are divided bio- 

 metrically by a very wide gap in the length frequency-curve, situated between 

 2G and 77 ix, into a larger and a smaller kind. The larger of these amphidiscs 

 I shall describe as mesamphidiscs, the smaller as micramphidiscs. 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 amphidiscs are so rare that 



