THE APODOUS HOLOTHURIANS 149 



cuticle. In the large intestine the epithelial lining is much the same, as it is 

 also in the cloaca, but in the latter the longitudinal muscle layer is functionally 

 replaced by about 20 isolated, irregularly arranged, small longitudinal muscles 

 lying outside the circular layer. The numerous strands connecting the cloaca 

 with the body-wall are similar in structure to the suspensors of the pharynx 

 already referred to. In Eupyrgus the walls of the cloaca are strengthened by 

 five large calcareous plates lying in the interradii (Plate XII, figs. 18, 26), while 

 in Aphelodactyla the cloacal wall contains irregularly branched calcareous jjar- 

 ticles (Plate X, fig. 6). Throughout the alimentary canal, wandering cells 

 in various forms (perhaps stages of develoj)ment) are abundant, particularly 

 in the internal epithelial layer. The mesentery which supports the alimentary 

 canal consists of a very thin sheet of connective tissue containing isolated 

 muscle fibers running in various directions and covered on both sides by the 

 epithelium of the body-cavity. That part which supports the pharynx, stomach, 

 and first part of the small intestine is attached to the body-wall on the right side 

 of the dorsal interradius ; the part which supports the forward-running section 

 of the small intestine passes over into the left dorsal interradius, close beside 

 the left dorsal radial muscle; when the large intestine bends backward, the 

 mesentery bends sharply to the left and crosses into the rifiht ventral inter- 

 radius, where sooner or later it terminates. In Caudina arenata it only gives 

 attachment to the large intestine for a short distance, but the latter is supported 

 by "two sheets of separate muscular strands," arising in the posterior part of 

 the right and left dorsal interradii, close to the right and left ventral radial 

 muscles. 



Respiratory trees (Plates X, fig. 1; XII, fig. 26). — The respiratory trees 

 consist of two branches, the right and left, which either arise separately from the 

 enlarged part of the cloaca or have a common opening into that organ; in the 

 latter case there may be quite a distinct common trunk of greater or less length. 

 The right branch consists of a single main tube, with more or less numerous 

 and conspicuous lateral out-growths, and may run forward clear to the calcareous 

 ring. The left branch, when fully developed, consists of two main tubes, one of 

 which is associated with the large intestine, and may be called ventral, while 

 the other lies in close connection with the blood plexus of the small intestine, and 

 may be called dorsal. Although the right branch is commonly larger than either 

 the left dorsal or left ventral alone, the left branch as a whole has a much 

 greater capacity than the right. In Eupyrgus (Plate XII, fig. 26), however, the 

 left branch is undivided and only equals the right in size, both being quite rudi- 

 mentary, and a similar condition exists in Gephyrothuria and Himasthlephora. 

 The histological structure of the respiratory trees is strikingly like that of the 

 intestine. The outermost layer is a thin epithelium of very flat, irregularly 

 polygonal, ciliated cells, followed by a thin layer of connective tissue. Then 



