58 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



4. The two rays between the bases of which the madre- 

 poric body is placed form the bivium. 



5. The anterior ray, together with one on each side of 

 it, make up the irivium. 



6. Notice that, while a line drawn through the anterior 

 ray and prolonged across the disc would pass through the 

 madreporic body and divide the animal into symmetrical 

 halves, this would not be true of a line through any other 

 ray. 



7. Make a sketch, showing these points. 



g, Examine a portion of the ab-oral surface with a lens, 

 and notice the pedicellarice ; small stony pincer-like struc- 

 tures, which are scattered over the spaces between the 

 ossicles, and are also found around the bases of the spines. 

 Each pedicellaria consists of a short stem and a pair of 

 movable jaws. 



h. With a sharp knife cut off one of the rays near its 

 union with the disc, and examining the cut surface, no- 

 tice ; — 



1. The ambulacrdl ossicles; two long slender plates 

 which occupy the centre of the oral surface, and form the 

 roof of the ambulacral furrow. Their lower ends are 

 widely separated, but the plates incline towards each 

 other, and their upper, slightly enlarged ends, meet upon 

 the median line of the ray, above the ambulacral furrow. 



2. The upper part of the ambulacral furrow is separated 

 from the lower open portion by a horizontal membrane- 

 ous partition, which may usually be found in a dried 

 specimen, running across from one ambulacral ossicle to 

 the other just below the point where they meet. The part 

 thus shut off contains the radiating ambulacral, or water- 

 tube. 



4. From the lower end of each ambulacral ossicle a 



