352 



ECHINODERMS OF THE BRITISH ISLES 



pieces. On the whole there is great variation in the structure 

 of the calcareous ring. In some deep-sea forms only the five 

 radials are developed. A single form, Pelagothuria, lacks the 

 calcareous ring completely. 



The alimentary canal is a long, usually simple tube ; generally 

 it forms a loop, passing first directly backwards along the mid- 

 dorsal interradius, then bending abruptly and passing forwards 

 in the left dorsal interradius towards the anterior end, where it 

 bends again, passing into the right ventral interradius, and then 

 straight to the posterior end, where it opens. In the Dendro- 

 chirotes, excepting the Psolids, the posterior part of the intestine, 

 however, lies in the left ventral interradius. It is suspended in 

 a mesentery which is attached to the body wall in the corre- 



FiG. 211. — Part of the calcareous ring of various Holothurians. x 2-5. 



(From Danmark's Fauna.) 



1. Thyone fusus. 2. Thuonidium pellucidum. 3. Psoitis sqiiamatus. i. Myriotrochus 



vitreus. R, Radial ; I, Interradial. 



sponding interradii. In the Elasipods, however, the mesenter^^ of 

 the posterior loop is attached in the right dorsal interradius. In 

 forms with a straight intestinal canal the mesentery still passes 

 from the mid-dorsal interradius in the anterior end to the right 

 ventral interradius in the posterior end. The rectum, or cloaca, 

 which plays an important role in respiration, is connected 

 with the body wall by numerous muscles. On the contrac- 

 tion of these muscles the cloaca is widely distended, and an 

 inward water - current produced ; when the muscles are relaxed 

 the water is pressed out again. From the cloaca issue in man}' 

 Holothurians a pair of more or less strongly branching, bush- 

 shaped, hollow organs, the respiratory trees (or water lungs), in 

 which the water alternately flows in or out by the widening or 

 contracting of the cloaca ; these are the special respiratory 



