156 INVERTEBRATE ZOOLOGY 
With strong scissors cut the body open by a longitudinal slit 
along the underside. Place it in a pan of water and with large 
pins pin down the two sides of the body wall on the right and 
left. Without cutting any of them, study the internal organs. 
Note the spacious body cavity and the long, coiled intestine 
which partly fills it. At the front end of the body and just be- 
hind the tentacles is the conspicuous calcareous ring, 2 more or 
less rigid cylinder containing five radial and five interradial plates, 
which surrounds the esophagus. Projecting from the hinder 
end of the calcareous ring will be seen two large cylindrical sacs, 
the Polian vesicles, and inserted in the ring are five prominent 
retractor muscles, by means of which the tentacles and the forward 
end of the body can be invaginated. Note the position of the 
genital gland and its duct. The gland is a thick bunch of slender 
filaments, in the forward part of the body cavity, joined with the 
body wall by a mesentery, which converge to the hinder end of 
the duct. This is a slender tube which passes forward on the 
upper side of the body cavity and opens to the outside between 
two tentacles on the upper side of the body. The sexes are 
separate; they are alike, however, in appearance. Note the respir- 
atory trees, the profusely branched organs which extend from the 
rectum throughout the entire length of the body cavity. 
Study the course of the digestive tract. The esophagus passes 
through the calcareous ring to the thick-walled, muscular stomach, 
which lies directly behind it. From the hinder end of the 
stomach the long, thin-walled intestine passes, with many loops 
and turns, to the short, wide rectum at the hinder end of the body. 
Trace the intestine carefully, but without cutting it, throughout 
its entire course and note the three mesenteries by which it is 
attached to the body wall. 
Note the thick muscular walls of the rectum and the muscle 
strands which join it with the body wall. The two branching 
respiratory trees spring from the forward end of the rectum and 
extend through the body cavity to its forward end. Observe 
