564 ECHINODERMATA HOLOTIIUROIDEA chap, xix 



oxygen into the fluid which fills tlie coelom. If a Sea-cucumber 

 be left in a limited quantity of water, it will sometimes direct 

 the posterior end upwards until it reaches the surface of the 

 liquid, and will pump air into the trees. Besides the trees, 

 other much shorter tubes open into the cloaca, termed the 

 Cuvierian organs. These tubes are really the modified basal 

 branches of the trees. They are unbranched, and their peri- 

 toneum consists of cells which secrete a slime which swells up 

 enormously on the addition of sea water. When the Cotton- 

 spinner is strongly irritated, it contracts all the muscles of the 

 body-wall, and these, acting on the incompressible fluid in the 

 body-cavity, transmit the pressure to the thin rectum, which 

 tears, and allows a portion of the viscera to be forced out. 

 The first parts to be rejected are the Cuvierian organs, and 

 the cells covering these absorb water, and their contained mucus 

 splits up into a tangle of white threads, in which an enemy may 

 be completely ensnared. A large lobster has been seen so 

 enveloped with this " cotton " as to be completely incapable of 

 motion. The origin of the name " Cotton-spinner " requires no 

 further elucidation. Such self-mutilation, even when it involves 

 not only the Cuvierian organs, but the trees and the whole of 

 the intestine, is not necessarily fatal. If the animal be left 

 alone, it can regenerate the whole of these organs. 



The water-vascular system in its general features resembles 

 that of the Echinoidea. We notice as its first striking peculi- 

 arity the modification of the stone-canal. This is often multi- 

 plied, as in the species {H. tubulosa) represented in Fig. 256, 

 where there are five ; but whether there is one or many, they do 

 not reach the body-wall, but end each in a swelling projecting 

 into and bathed by the coelomic fluid. These swellings are 

 termed "internal madreporites." They are pierced by numerous 

 fine ciliated canals, which lead into a space from which the 

 stone-canal takes its origin. Both stone-canal and madreporite 

 (especially the latter) are stiffened by the deposition of carbonate 

 of lime. In the young Holothurian there is a single ciliated 

 pore-canal opening to the exterior and leading into a thin-walled 

 axial sinus, which, as Bury ^ has shown, is later converted into 

 the internal madreporite ; the pore-canal, which represents the 

 external madreporite of other Echinoderms, disappearing at the 

 ^ "The Metamorphosis of Echinoderms, " Quart. J. Micr. Sci. xxxviii. 1896, p. 53. 



