A SEA CUCUMBER 125 



Note the spacious body cavity and the long, coiled intestine 

 which partly fills it. At the front end of the body and just behind 

 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 re- 

 tractor 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 sepa- 

 rate ; they are alike, however, in appearance. Note the respira- 

 tory 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 en- 

 tire 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 care- 

 fully their extent. It is by the periodic dilation and contraction 

 of the muscular walls of the rectum that water is alternately taken 

 into the rectum and the respiratory trees and expelled from them, 

 and respiration thus carried on. 



Exercise 2. Draw a diagrammatic view of the opened animal on a 

 large scale, showing the organs observed. Carefully label all. 



