ARBACIA OR STRONGYLOCENTROTUS, THYONE 227 



so well concealed that they cannot be satisfactorily studied 

 in their native places. It is desirable to visit places where 

 they occur and to observe the parts visible above the mud. 

 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. How many tentacles? 

 Arrangement? To what structures in the sea urchin do they 

 probably correspond? 



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? 



Kill the specimen by catching it with strong forceps be- 

 hind the mouth, when the tentacles are expanded, and holding 

 it in hot water. 1 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. 



2. The suckers are less abundant on the dorsal (upper) 

 surface than on the ventral. 



3. A small papilla is to be found on the dorsal surface, 

 between the tentacles. On it is the genital opening. This 

 will be referred to again. 



Make a drawing of the animal as seen from the side, in- 

 dicating all of the points of structure that have been seen. 



With a pair of scissors, open the animal longitudinally 

 along the middle of the ventral (lower) surface. 



Digestive System. — Note: 1. The delicate, perforated 



mesentery, which attaches digestive tract to body wall. 



1 Specimens that do not expand may be injected with a saturated 

 solution of chloretone (saturated by heating). After the animal re- 

 laxes the tentacles may be pushed out. Then kill in hot water or dis- 

 sect immediately. 



