THYONE. 75 



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 

 branched and project far forward into the body-cavity. Can you 

 determine how they are filled with water and how the water is ex- 

 pelled? With a pipet inject them with starch-mass. The strong 

 jets of water ejected by the living specimen were thrown from 

 these tubes. Can you understand how they serve for respira- 

 tion? The walls of the tubes composing the trees are glandular 

 and may thus serve to excrete wastes. Notice the muscles that 

 radiate from the walls of the cloaca to the body-wall. What 

 is their function? 



Make a drawing of the cloaca and rspiratory trees. 



Reproductive System. The single gonad (ovary or testis) 

 occupies a median dorsal position in the anterior part of the 



