66 



the fourth to the end of the arm (except, of course, as regards 

 their anatomical and histological structure); they may, however, 

 prove to be of no smaller importance from a systematic point 

 of view than are the three lowermost ones — if it be allowed to 

 conclude from the fact that they are so in these two groups. 



Ä. Eschrichti. The joints are rather thick and robust, on 

 the interior side with a sharp, thin keel, well marked off from 

 the thick part of the joint; only on the last joint no such keel 

 is distinctly seen. The last 6 — 7 joints have their outer side 

 provided with numerous spines or hooks, especially the last 

 joint has 2 — 3 groups of large hooks. (PI. I. Fig. 5). 



The covering-plates («ambulacrals») (PI. 1. Fig. 6) are large, 

 cribrous, of very irregular outline; the lower part is much the 

 larger and may consist of two separate plates, lying very clo- 

 sely together; they unite with the neighbouring plates and thus 

 form a complete covering on the side of the ambulacral groove, 

 interrupted only by the narrow spaces in which the sacculi are 

 lodged, sacculi and covering-plates alternating regularly. The 

 upper part of the covering-plate may be more or less separate, 

 forming a very irregular plate, whose adorai side is bent some- 

 what inwards; this part is much smaller than the lower part, 

 the plates being thus widely separated from each other. The 

 connection between the two parts may be represented by a 

 single rod, which forms, however, mostly a more or less exten- 

 sive network on its aboral, but not on its adorai side. On the 

 genital pinnulae there are formed a number of thin irregular 

 plates, covering the widened portion between the joints and 

 the «ambulacral') plates. When dried these pinnulae thus show 

 a very distinct, irregular plating. (PI. 1. Fig. 4). It may yet be 

 added about the pinnulae of this species that the ambulacral 

 groove persists almost to the end, though, of course, rather 

 rudimentary; the covering-plates are not found on the last 

 6 — 7 joints, but the sacculi may proceed almost to the last 

 joint. (PI. I. Fig. 5). 



