268 ECHINODERMS. 



drawing the body of the animal forward with a slow, 

 gliding motion. 



2. Turn an active starfish upside down, and observe 

 how the arms are bent, and the tube feet used in righting 

 itself. 



3. Place a live specimen in a shallow dish of sea water, 

 and with a lens examine it very near the surface of the 

 water, to find about the bases of the rigid, blunt, conspic- 

 uous spines numerous smaller ones, some of which are two- 

 parted at the tip (pedicellarice), and which are continually 

 snapping. These are believed to keep the surface clean 

 by preventing a deposition of sediment. 



4. Observe also between the blunt spines numerous 

 conic or cylindric soft membranous processes, protruding 

 sacs filled with fluid (branchial tentacles). Test the sen- 

 sitiveness of these and of the tube feet to touch. 



External Features. Use either alcoholic l or freshly 

 chloroformed specimens, the latter when readily obtain- 

 able. Observe : 



1. That the flattened body consists of a disk and a 

 number of radiating arms. Are the number and size of 

 these the same in all specimens ? 



2. That it presents two well-marked surfaces : 



(a) A flat surface on which it crawls, in the center of 

 which is the mouth, and from this fact called the oral surface. 

 (6) A concave upper aboral surface. 

 On the oral surface note : 



1. The round mouth, with the thin crumpled walls of 

 the stomach often seen protruding from it. 



2. The deep and wide grooves extending from the 

 mouth to the tip of each arm. As these lodge the tube 

 feet, or ambulacra, they are called ambulacral grooves. 



1 If alcoholic specimens have acquired a disagreeable smell, this may 

 be overcome by adding a few drops of oil cassia to the alcohol. 



