68 ECHINODERMATA. 



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 carti- 

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

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

 sea-urchin lantern. 



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. 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 respiration? The walls 

 of the tubes composing the trees are glandular and may thus 

 serve to excrete wastes. 



Make a drawing of the cloaca and respiratory trees. 



Reproductive System. — The single gonad (ovary or testis) 

 occupies a median dorsal position in the anterior part of the 

 body-cavity. It is composed of a multitude of filaments, which 

 join to make a brush. This brush projects backward into the 

 body-cavity. The duct of the organ lies along the dorsal mid- 

 line, between the two muscle bands, and leads to the opening 



