NO. 19.] ECHINODERMS OF CONNECTICUT. 119 



teeth ", situated in the five radii. There are also five groups of 

 slender, sensory papillae surrounding the opening. 



Respiratory Trees.*— The anterior end of the cloaca leads 

 forward into a broad sac, which divides into a pair of very thin- 

 walled tubes, the so-called respiratory trees. These are profusely 

 branched (Plate XXX), and extend forward nearly to the 

 anterior end of the body. Their very numerous broad-lobed rami- 

 fications (which are made too slender in the figure) expose a 

 vast amount of surface to the coelomic fluids. 



The musculature described above enables the cloaca to be 

 widely opened and filled with the external sea water. By closing 

 the cloacal aperture this, water is forced up into the respiratory 

 trees under considerable pressure. In this way pure water is 

 supplied to the branches of the respiratory trees, and because of 

 the pressure a portion of the water is forced through the walls 

 of the branches and enters the ccelom. This cavity is thus con- 

 stantly filled with water, and the body walls kept tense and firm. 



Such a continuous supply is necessary to replace the loss 

 which constantly occurs by diffusion through the tentacles and 

 tube-feet during the active movements of the animal. 



By constantly pumping the water in and out of the cloaca the 

 body receives a constant supply of pure water, bringing in the 

 oxygen necessary for life, and carrying out the waste formed in 

 the metabolic processes. 



When the animal is disturbed and contracts its body violently, 

 at the same time withdrawing its tentacles, the water in the 

 respiratory trees is driven out of the cloacal aperture with such 

 force that it forms a jet which may be ejected for a considerable 

 distance. 



In Synapta the respiratory trees are wanting. Instead, there 

 are numerous ciliated funnels attached to the left dorsal mesentery 

 which probably have an excretory function. 



Water-vascular System, f — The ring canal at the base of the 

 tentacles (Plate XXX) connects with radial canals which ex- 

 tend to the posterior end of the body between the two bands of 



* By injecting colored starch paste through the cloaca, the respiratory trees 

 may be made to show all their branches with great distinctness. 



t It is possible to inject the water-vascular system through one of the Polian 

 vesicles with a colored fluid, such as Prussian blue solution. By this method the 

 radial canals with their branches may be rendered quite conspicuous. 



