VIII ECHIN0D:ERMATA—THE C(EL0MIC cavities 439 



The lymph gills, branchial vesicles or papulse, which are only 

 found in the Asteroids, deserve attention. These are small vesicular 

 bulgings of the body v^^all, which occur in great numl)ers between the 

 skeletal plates. On these bulgings, the body wall is very thin, and, in 

 order to facilitate osmosis, devoid of calcareous deposits. It consists 

 of layers similar to those found in other parts of the body : an outer, 

 strongly ciliated and glandular epithelium ; a middle layer of con- 

 nective tissue, containing longitudinal and circular muscle fibres ; and 

 an inner ciliated epithelium, which is nothing else than the endothelium 

 of the body cavity. The cavities of the papula^ are merely diverticula 

 of the body cavity which bulge out the much-thinned body wall. 



The papiiliii are sensitive, and contract at the slightest touch. 



In Asteroids with very thick body wall, diverticula of the body cavity force their 

 way into it, branching on their way to the surface. On reaching this latter, each 

 branch enters a branchial vesicle. 



In certain forms, a single diverticulum traversing the body wall supplies a whole 

 group of branchial vesicles. 



A constant streaming to and fro of the Ijody fluid can easily be observed in the 

 branchial vesicles. 



Each branchial diverticulum of tlie liody cavity is surrounded, in the connective 

 tissue layer of the body wall, by a circular lacuna. 



The branchial vesicles occur both on the arms and on the disc. In the Phanero- 

 zonia they are found only on the upper side of the body ; in the Cryptozonia, on the 

 contrary, they occur on the sides of the arms as well, and on the lower (oral) side of 

 the body. 



4. Ophiuroidea. — The body cavity of the disc is much limited 

 by the digestive sac and the bursaj. Filaments and bands of con- 

 nective tissue, covered with endothelium, traverse it at irregular 

 intervals, and connect the viscera with the body wall. 



o. Crinoidea. — The body cavity of the calyx is almost completely 

 filled with bands, trabeculie, filaments, etc., of connective tissue covered 

 with endothelium, which together form a spongy network, and often 

 become calcified. In this network a sac-like membrane becomes 

 differentiated, which divides the body cavity into a central and a 

 peripheral space. The central space, which contains the intestine, is 

 known as the peri-intestinal cavity ; the peripheral, as the subtegu- 

 mentary cavity. The peri-intestinal cavity, again, contains another 

 separate portion of the ccelom, the axial body cavity, round which the 

 intestine coils itself. This latter cavity encloses the genital stolon 

 and communicates, on the one hand, with the five chambers of the 

 chambered organ which lies in the apex of the calyx, and through it 

 with the coelomic canals of the stalk and the cirri ; and on the other, 

 Avith the oral or subtentacular canals of the arms. The peri-intestinal 

 cavitjr, on the contrary, is continued into the dorsal or apical brachial 

 canals. 



