154 PRACTICAL ZOOLOGY. 



5. The soft cylindrical projections along the median tract 

 of the oral surface of each ray, are the ambulacra 01 

 tube feet. Remove one of them and examine it with 

 care. Note the arrangement of the series. 



6. Press apart the tube feet and find running along the 

 median line of the ambulacral groove, a yellowish or 

 whitish ridge, the nerve of the ray. Trace it to the 

 soft membrane bordering the mouth, the peristome, 

 and find the nerve ring around the mouth. 



7. Trace the nerves also to their outer ends and find a 

 reddish or yellowish elevation, the eye-spot, borne at 

 the base of a median terminal tentacle, resembling a 

 tube foot. 



8. The eye-spot is borne on a distinct, but minute, plate. 

 Compare young and old specimens to see that what- 

 ever the size, this single ocular plate with its eye-spot 

 is always at the end of the ray. Count the ambula- 

 cral plates in a short and in a long ray. Where do the 

 new plates develop ? 



DISSECTION OP THE STABEISH. 



1. The ray opposite the madreporic body is the anterior 

 ray. Cut through its aboral wall near the outer end, 

 and from this point cut along the upper part of each 

 side of the ray, an inch or two toward the disk ; raise 

 the flap thus freed, and, avoiding internal organs, 

 continue the cut on each side to the disk. 



2. Attached to the aboral wall find a pair of elongated, 

 branched bodies, the hepatic caeca. Note how each 

 csecum is held in place by the thin mesentery. 



3. Along the middle line of the aboral wall, inside, is a 

 yellowish streak, the extensor muscle of the ray; 

 with forceps prove its general structure. 



