THYONE. 81 



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. 1 Note that: 



1. The body is covered with papilliform ambulacra! 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, be- 

 tween 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, indicat- 

 ing 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. 1. Note the delicate perforated mesen- 

 tery, which attaches it to the walls of the body. 



2. The esophagus, leading from the mouth through a 

 calcareous structure, which recalls the lantern of the sea-urchin. 

 Examine and see if the arrangement is similar to that of the 

 sea-urchin lantern. The muscles for the retraction of the 

 lantern are frequently torn from their attachments at one end. 



3. The thin-walled and enlarged stomach. 



4. The coiled intestine, which leads to the cloaca. 

 Draw the alimentary canal in position. 



Cut the alimentary canal just in front of the stomach, and 

 close to the cloaca, and as you remove it notice the blood-vessel 

 that runs along the intestine. 



Respiratory and Excretory System. Arising laterally from 

 either side of the cloaca are the two respiratory trees. They are 



*. Specimens that do not expand may be injected with a saturated 

 solution of chlorotone (saturated by heating). After the animal relaxes 

 the tentacles may be pushed out. Then kill in hot water or dissect im- 

 mediately. 

 6 



