406 Mr. W. P. Sladen on the Structure of 



Extending from the mouth -slits along the median line of 

 the radii are a series of large quadrangular plates, occupying 

 the position of under arm-plates in an Ophiuran (PI. XX. 

 fig. 6). The first or innermost under arm-plate is much larger 

 than any of the succeeding ones, elongate and somewhat 

 escutcheon-shaped inwardly, but having two broad alas or 

 extensions produced with a graceful curve from the corners of 

 the outer margin, the whole plate bearing a fanciful resem- 

 blance to the gorget of ancient armour. The succeeding 

 under arm-plates are subquadrate, and, being bounded late- 

 rally by incurved lines, resemble vertebras in outline ; the 

 number included within the disk is nine or ten, of which the 

 two or three terminal ones diminish very rapidly in breadth, 

 the passage of these into the true under arm-plates of the 

 aborted ray being clearly traceable — a subject to which 

 attention will be directed presently. 



The space occupied by the ambulacral system is very con- 

 siderable, forming in each radius a petaloid area, down the 

 middle of which runs the series of under arm-plates above 

 described (PI. XX. fig. G). The entire intermediate space 

 between two neighbouring radii is covered with a tessellated 

 scaly membrane, composed of small, uniform, hexagonal 

 plates. The tessellse, which form a more or less straight line 

 bordering on the pore-area, diminish in size as they approach 

 the edge of the disk, and at the same time become somewhat 

 rounded in contour ; and the plating terminating altogether at 

 a short distance from the margin of the disk, a border is left 

 around the test exposing a portion of the lateral or side arm- 

 plates, in breadth rather greater than the length of the longest 

 spines of the marginal fringe. 



Returning now to the ambulacra. A pair of tentacles 

 accompanies each under arm-plate, one on either side ; and 

 the neighbouring tentacles or ambulacral feet are separated 

 from those of the next joint by a thin straight partition or 

 septum, which extends somewhat upward into the disk or 

 body in a manner suggestive of the arrangement of the 

 ambulacral plates in certain Asteroids. Owing to the promi- 

 nence of these septa, which extend between the outer angles 

 of the under arm-plates and the margin of the interradial 

 plating, they have the appearance of cutting up the ambu- 

 lacral area into square compartments ; over these is stretched 

 a thin membranous skin, which is punctured with a large 

 round pore for the passage of the ambulacral tentacle, the 

 orifice being frequently very little less in diameter than the 

 length of the under arm-plate itself (fig. 6). Judging from 

 the appearance of the specimen, the tentacles were, in all 



