ECHINODERMATA 209 



Make an outline drawing of each surface, filling in the de- 

 tails of the disc and one arm and showing the points above de- 

 termined. 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 caeca; extent, number, and attachment. 



Detach the hepatic caeca from the aboral wall by breaking the mesenteries, 

 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 between it and the 

 centre of the disc, being careful to disturb none of the soft parts. A few good, 

 permanent dissections, to be used as demonstrations, will be of value. 

 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 diflScult of demonstration.) 



Rectal diverticula? number and position? 



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



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 arrange- 

 ment? 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 ambulacra 



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 reproductive bodies in a 

 second, and the ampullae and retractor muscles in a third; (2) a trans- 

 verse section of an arm about midway between its ends; and (3) a sagittal 

 section of an arm continued through the disc. 



4. Ambulacral system. 



In a specimen from which the preceding organs have been removed, make a 

 transverse section of an arm about an inch from the disc. Find the radial water 

 canal, a small tube lying just outside the skeleton in the ambulacral groove. Force 

 air into it with a blow-pipe, or inject a colored solution with a hypodermic syringe. 

 What other structures are afifected? Trace connection between radial canal, 

 ampullcB, and ambulacral feet. Compare the number of ampullae and the number 

 of feet. Follow the radial canal toward the disc. How does it terminate? 



From the madreporic body trace the S-shaped stone canal toward the oral 

 surface. How does it terminate? 

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