50 ' PEOl". A. DENDT ON THE 



most. The strong curvature of the spokes is well seen, and also 

 the central cavity, with the cross on one side and the small 

 round hole on the other. 



We may now pass on to describe the growth changes of the 

 wheel. It has long been known that it first appears as a six- 

 rayed cross, and I find that this cross persists as the cross on 

 the inner face of the fully-formed spicule. 



In the earliest stage which I have met with (PI. 3. fig. 3) there is 

 already a thickening at the end of each ray of the primary cross 

 and on its outer face. These thickenings increase in size and 

 extend both outwards (towards the outer surface of the integu- 

 ment), and centripetally (towards the centre o£ the cross), till 

 they meet together (figs. 3 a, 4), leaving a hexagonal aperture 

 in the middle. The lines of junction between the adjacent 

 thickenings are at first clearly visible (figs. 4, .5), but they 

 presently disappear, and at the same time the margin of the 

 aperture becomes rounded off. Thus is formed the small round 

 hole seen in the middle of the outer face of the fully-formed 

 spicule. 



Meanwhile the thickening of each primary ray has also been 

 extending centrifugally to form one of the six spokes of the 

 wheel, each of which is for some time irregularly bifid at its 

 extremity (figs. 4, 5). When the spokes have attained their 

 full length their ends thicken laterally and on the outer aspect, 

 and these thickenings, meeting and fusing, form the rim of the 

 wheel, in the manner well known for other species.* 



It is the formation of the first, centripetal thickenings and the 

 consequent development of the small round hole on the outer 

 face of the spicule which have not, so far as I am able to 

 ascertain, been hitherto described. 



EXPLANATION OF TFE PLATES. 

 Plate 3. 



Figs. 1-8. Chirodota dunedinensis. 

 Figs. 1-2. Wheels, viewed from the two surfaces and represented as opaque 

 objects. (Zeiss D, Oc. 2, Camera.) 

 Fig. 2 a. Optical vertical section of wheel. 



3. Early stage in the development of the wheel, the ends of the 

 primary rays just commencing to thicken. (Zeiss D, Oc. 2, 

 Camera.) 



* Cf. Ludwig in Bronn's Klass. u. Ordnung. d. Thier-Eeichs, Ed. ii. Abth. 3 

 (Echinod.), p. 55, and also Kishinouye, Zool. Anz. Bd. xvii. p. 146. 



