ECHINODERMATA. 2OI 



Pedicellariae (at the base of the spines) ; papulae (soft 



bodies among the spines). Examine with lens. 

 Make an outline drawing of each surface, filling in the 

 details of the disc and one arm and showing the points above 

 determined. Sketch one of each of the various classes of 

 spines in profile. 



3. Organs of the body-cavity. 



Using alcoholic or other moist preparations, cut into one side of an 

 arm of the trivium, making an incision from near the tip almost to the 

 disc. Cut across the back of the arm near the tip and make a similar 

 incision on the other side. Lift the flap thus separated and notice the 

 organs attached to it. The material should be dissected under water or 50 

 per cent, alcohol, or kept moistened therewith. 

 Hepatic cseca ; extent, number, and attachment. 



Detach the hepatic cseca from the aboral wall by breaking the mesen- 

 teries, and treat all the arms of the trivium as above. 



Carefully connect the incisions across the interradii and remove the 

 entire aboral wall except that around the madreporic body and that be- 

 tween it and the centre of the disc, being careful to disturb none of the 

 soft parts. If material is scarce the teacher should make a few dissec- 

 tions to be used as demonstrations. 

 Notice : 



Body-cavity, its extent and contents. 



Stomach: pyloric (aboral) portion; shape, position. Are the hepatic 

 caeca connected with it? Verify. (The stomach opens aborally into 

 a small, short rectum and anus usually very difficult of demon- 

 stration.) Rectal diverticula? number and position? 

 Cardiac (oral) portion of stomach; pouches, number and form; re- 

 tractor muscles, attached to the floor of the arms. 

 Mouth ; peristome. 



Remove the hepatic caeca from one arm and find the genital glands, 



which lie in the floor of the body-cavity. What is their number 



and arrangement ? At what point do they connect with the body 



wall? Can you prove that they communicate with the exterior? 



Ampullae (on ventral floor) : determine if they connect with ambu- 



lacral feet. 



Make three diagrams showing the position of the organs thus far 

 studied: (i) the aboral surface with the wall removed, showing 

 the stomach in the disc, the hepatic caeca in one arm, the repro- 

 ductive bodies in a second, and the ampullae and retractor muscles 

 in a third; (2) a transverse section of an arm about midway be- 

 tween its ends; and (3) a sagittal section of an arm continued 

 through the disc. 



