ASTERIAS. 71 



the body integument on the aboral surface. They serve as 

 respiratory organs and probably also have an excretory func- 

 tion. The phagocjrtic nature of the cells of the coelomic fluid 

 may be studied by simple methods reported by Kindred. 

 (See reference below.) 



5. Stroke the starfish with a camel' s-hair brush and notice 

 how the hairs are caught. Can you determine by what and 

 how they are held? With a hand-lens examine around the bases 

 of the spines, and see the arrangement of the pedicellarice. Their 

 function is obscure, but they enable the starfish to hold small 

 objects firmly and they may be of service in dealing with possi- 

 ble surface parasites. 



6. Remove some of the pedicellarioe with a scalpel and ex- 

 amine them under the microscope. Do you find more than one 

 kind? 



Draw a pedicellaria. 



Internal Structure. — Make the dissection under water, and in 

 cutting through the integument he careful not to injure the 

 underlying soft parts. 



With strong scissors cut through the aboral body-wall near 

 the tips of the rays of the trivium. Carry the cuts forward 

 along the sides of the rays to the disk. The cavity thus opened 

 is the ccelom or body cavity. 



Lift up the integument at the tip of each arm and carefully 

 snip away the mesenteries which attach the organs to it. Cut 

 the membranes that extend into the disk opposite the junc- 

 tions of the arms, and remove the three-rayed flap of integu- 

 ment thus freed, cutting as close as possible to the madreporite, 

 but leaving this in place. 



Digestive System. — In studying this system you should con- 

 stantly bear in mind the pecuHar method by which the animal 



Irving, L.: Ciliary Currents in the Starfish. J. E. Z., vol. 41, 1924. 



Irving, L. : Regulation of the pH Concentration and Its Relation to Metab- 

 olism and Respiration in the Starfish. Jour. General Physiology, 

 November 20, 1926. 



Kindred, J. E. : The Cellular Elements of the Perivisceral Fluid of Echino- 

 derms. Biol. Bui., vol. 46, 1924. 



