THE APODOUS HOLOTHTJKIANS 53 



surface, but much more flattened. Whether they are provided with cilia or not 

 is a disputed point. Hamann ( '84) asserts that cilia are present, but other 

 investigators have failed to find them. My own observations lead me to be- 

 lieve that cilia are not usually present on the cells of this inner epithelium. 



BODY-CAVITY. The body-cavity of the Synaptids is always spacious, and 

 extends from the oral disc to the posterior tip of the body without any marked 

 separation into parts, although the portion around the oesophagus has had a 

 different origin from the rest in the development of the animal. Longitudi- 

 nally the cavity is divided on the dorsal side by the mesentery, which supports 

 the intestine, and anteriorly it is traversed by strands or folds of tissue 

 which support the oesophagus. All parts of the body-cavity are lined with the 

 thin, flattened epithelium, already described as forming the innermost layer 

 of the body-wall. The fluid contained in the body-cavity is largely water, but 

 contains albuminous material and corpuscle-like cells. It is probably identical 

 with the fluid in the water-vascular system, but may contain more water. So 

 far as known, the body-cavity is always in communication with the water- 

 vascular system through the madrepore bodies and stone-canal. In Synaptula 

 hydriformis and probably in Chiridota rotifera the body-cavity is in commu- 

 nication with the outside through openings in the wall of the rectum and at 

 times through rupture of the body-wall near the anus. The former openings 

 seem to be permanent, but the latter are only formed when the young are born 

 and apparently heal up thereafter. In the viviparous species just mentioned, 

 the body-cavity fluid must contain nutritious material which serves as food 

 for the young, as the latter grow to considerable size (10 mm. long or more) 

 before birth. 



CALCAREOUS RING. Surrounding the oesophagus in Synaptids, as in all other 

 holothurians, is a circular ring, made up of more or less distinct calcareous 

 plates. Of these, five lie at the anterior ends of the longitudinal muscle bands, 

 which are attached to them, and are called the radial pieces, while the remain- 

 der are called interradial. These plates arise from calcareous bars (Plate 

 VI, fig. 9) which branch more or less irregularly, the branches anastomosing 

 and fusing until solid plates are formed. The plates are generally more or 

 less quadrilateral, but may be higher than wide (Synaptula hydriformis, Plate 

 VI, figs. 7 and 8) or approximately square (Plate V, fig. 14) or wider than 

 high (Leptosynapta roseola, Plate V, fig. 16). Usually the posterior margin 

 of each plate is slightly concave or even notched, but it is never forked or 

 deeply divided. The anterior margin may be straight or convex, or distinctly 

 pointed (Plate IV, fig. 2), and frequently the radial pieces are either pierced 

 or notched for the passage of the radial nerves; in Labidoplax digitata only 

 the three ventral radial pieces are so pierced. Occasionally the radial pieces 

 of the ventral side are better developed than those of the dorsal, or the reverse 



