80 ECHINODERMATA. 



1. A nerve ring that encircles the esophagus at a point just 

 above the mouth. 



2. Five radial nerves that pass from the ring, along the in- 

 side of the ambulacral areas of the test, to the ocular plates. 



The radial water tubes will be found in sections adjacent 

 to the radial nerves. The two are separated only by a narrow 

 space, the pseudohsemal canal. Between the radial nerves 

 and the tissue of the test there is another narrow cavity, the 

 epineural sinus. 



If time permits, students will find a dissection of the sand- 

 dollar, Echinarachnius, valuable for purposes of comparison. 

 Special notes will not be necessary. Its shape and restricted 

 ambulacral areas should be studied in the light of its habits 

 and food-supply. How does the animal move? 



MacBride: Cambridge Natural History, Echinodermata. 

 Tennent: Variation in Echinoid Plutei. Jour. Exp. Zool., 9, 1910. 

 Von Uexhiill: Die Physiologie des Deeigelstachels. Zeit. f. Biol., 39. 



: Ueber die Function der Polischen Blasen am Kauapparat der regu- 



laren Seeigel. Mitth. Zool. Stat. NeapeL, 12, 1897. 



HOLOTHUROIDEA. 

 THYONE. (Sea-Cacumber.) 



These animals may be found in protected and usually muddy 

 places, concealed in eel-grass. They are generally so effectually 

 concealed that they cannot be satisfactorily studied in their 

 native places. It is desirable to visit places where they occur 

 and find specimens by feeling for them near the bottom. It 

 is then possible to realize the life for which they are adapted. 



Examine a living expanded specimen in an aquarium (taking 

 care not to disturb it) and note: 



1. How the tentacles are used. What kind of food would 

 it get by this means? Compare the method of food-getting 

 with the starfish and sea-urchin. 



2. The respiratory movements of the body. Notice the 

 strength of the current of water ejected. 



3. The general shape of the body when expanded. Does 

 it seem to rest on a particular side? 



