148 THE APODOUS HOLOTHURIANS 



terior end of the body, 1)nt apparently none of these are now in connection with 

 the radial canals. The fluid contained in the water-vascular system is prob- 

 ably not essentially different from that in the body-cavity, since there is con- 

 stant communication possible through the madreporite, but there are a much 

 larger number of corpuscles and wandering cells in the contents of the water- 

 vascular system. The corpuscles are yellowish when seen singly, but in mass 

 appear to be red. The lumen of the polian vessel is sometimes nearly filled 

 with dark-brown, apparently dead, wandering cells. 



Alimentary canal (Plate X, fig. 1). — The mouth is a circular opening in the 

 center of the oral disc at the anterior end of the body and is connected with the 

 stomach by a straight, rather short tube, which is commonly called the phariinx, 

 although it corresponds to the esophagus of the Synaptid^^ and is not specially 

 modified in any way. The stomach is simply a slight enlargement of the ali- 

 mentary canal and is relatively short (say one-tenth of the total length). It 

 opens posteriorly into the small intestine, which extends backward only a short 

 distance and then bends abruptly forward for a greater or less distance, finally 

 turning backward, ventral to the other viscera, as a large intestine, which runs 

 to the |iosterior tip of the body. The hinder part of the large intestine is rather 

 abru]itly enlarged whei'e it receives the resjiiratory trees and forms the cloaca, 

 the length of which is closely associated with the development of the caudal ap- 

 pendage. The various sections of the canal as here given are not, as a rule, 

 sharply set off from each other, although, according to Gerould, they can be 

 easily distinguished in living or freshly killed sjiecimens of Caudina. They all 

 have a very similar histological structure, consisting of an outer epithelium 

 continuous with that of the body-cavity, a thin layer of connective tissue, a 

 muscular layer, a thick layer of connective tissue, and a lining epithelium. The 

 relative development of these layers differs in the different parts of the canal. 

 The pharynx when relaxed is somewhat larger at the mouth thaTi where it joins 

 the stomach, but may be wholly closed anteriorly by the well-developed sphincter 

 muscle, composed of circular muscle fibers which gradually thin out on the oral 

 disc. Posteriorly, the muscle layer of the ]iharynx contains longitudinal fibers 

 within the circular series; the epithelial lining of the pharynx contains many 

 gland cells, is covered by a delicate cuticle, and is thrown into longitudinal folds. 

 The pharynx is held in i)osition by 10 longitudinal series of connective-tissue 

 strands containing muscle fibers; these unite it with the inner surface of the 

 calcareous ring, and are called suspensors by Gerould. In the wall of the 

 stomach the longitudinal muscle layer is very well developed, as is the circular 

 series, while the lining epithelium, consisting of high columnar and gland cells, 

 is covered with a delicate cuticle. 



In the small intestine the longitudinal muscle layer is practically wanting, 

 while the lining epithelium is remarkable for the absence of gland cells and 



