60 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



on opposite sides of the ray, and a very slight power of 

 motion between those of the same side. 

 ■ 2. On each side of the ridge is an area marked by two 

 rows of small round openings, and also by fine parallel 

 lines continuous with the transverse furrows of the ridge, 

 and therefore at right angles to the long axis of the ray. 

 This is the area of the ambulacral ossicles. Comparison 

 of the surface view with the sectional view shows that 

 each ossicle is a thin, vertically flattened plate, with its long 

 axis at right angles to the long axis of the ray. It is 

 joined by its inner end to the corresponding ossicle of the 

 other side, and by its flat faces to the plates before and 

 behind it on the same side. In the description of these 

 plates the face nearest the base of the ray Avill be called 

 the proximal and that nearest the tip the distal ; the end 

 nearest the middle of the ray the central, and that farthest 

 from the middle line the perii)heral. On each side of each 

 plate there is a perpendicular groove, and the grooves of 

 adjacent plates meet so as to surround tubular spaces 

 which run from the interior of the ray to the lower sur- 

 face. 



The inner ends of these tubes, which are the ambulacral 

 pores, are seen on each side of the vertebral ridge. At 

 first sight they seem to be arranged in a double row, but 

 a more careful examination shows that there is only one 

 pore between each pair of ambulacral plates, but that they 

 are alternately central and peripheral, thus forming a 

 "zigzag" instead of a straight line. 



Each plate has one groove on each side, one near the 

 peripheral end and one near the central end, and the posi- 

 tion of the grooves alternates in adjacent plates, so that if 

 the groove on the distal side of one plate is near the peri- 

 pheral end, the groove on the proximal side of the next 



