74 ECHINODERMATA. 



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. 



Von Uexhull: Die Physiologic des Seeigelstachels. 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-Cucumber.) 



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 living expanded specimens in an aquarium (taking 

 care not to disturb them) 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 ? 



4. The number and arrangement of the tentacles. To what 

 do they probably correspond in the sea-urchin? 



Kill the specimen by catching it with strong forceps behind 

 the mouth, when the tentacles are expanded, and holding it in 

 hot water. Note that : 



1. The body is covered with papilliform ambulacral feet. 

 It is possible in some cases to see that they are arranged in five 

 broad, longitudinal bands. 



