228 ECHINODERMATA 



2. The esophagus, leading from the mouth through a cal- 

 careous 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 matter is expelled? With a pipette 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. 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 respiratory trees. 



Reproductive System. — The single gonad (ovary or tes- 

 tis) 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 midline, between the right and left dorsal muscle 

 bands, and leads to the opening upon the small papilla near 

 the mouth that has already been noticed. 



Water-vascular System. — 1. The circular canal can be 

 found in favorable specimens, surrounding the deeper portions 

 of the esophagus. It gives rise to one or two Polian vesicles, 

 which are very large and hang down into the body cavity. 



