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, a more or 

 less rigid cylinder containing five radial and five interradial plates, 

 which surrounds the oesophagus. 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 oesophagus 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 





