42 
ZOOLOGICAL EXERCISES. 
is connected with the rest of the stomach below bya 
narrow isthmus. The stomach is a pale coloured— 
generally greenish or greenish-yellow—much folded 
bag, which sends out short prolongations—the cardiac 
sacs—into each ray. ach cardiac sac is connected 
with the ambulacral ossicles by radiating mesenteric 
folds, which hold it in its place. The stomach is 
connected with the mouth by a broad gullet. Examine 
portions of the radial czeca and of the stomach with 
the quarter-inch objective. Notice the currents 
caused by the cilia. With careful focussing, the cilia 
themselves may be seen. 
10. With the scissors and forceps carefully remove the 
Lhe = 
whole of the alimentary system. Pour off the water 
out of the saucer and put in clean. Observe the 
hyaline, or greenish, polian vesicles, one in each inter- 
radial angle, except the one occupied by the stone 
canal, It is rarely that all four can be seen, some 
being generally empty. 
race down the white or yellowish stone canal from 
the madreporic tubercle to the opening in the central 
ambulacral ossicles through which it disappears. 
Alongside it lies a vessel of the vascular system, the 
so-called heart. Cut out the stone canal, and detach 
it from the madreporic tubercle. Examine it with 
the microscope; notice, with the quarter-inch, the cal- 
careous plates in its walls. Place the madrepori¢ 
tubercle on a cork, and cut a section from the middle. 
Examine with the microscope, and notice the pores. 
12, Examine the ampulle, or ambulacral vesicles, of the 
pedicels, or tube feet, arranged in two rows on each 
side of the ambulacral ossicles. Scrape some away, 
and observe the two rows of openings through which 
