HOLOTHUROIDEA. 251 



In Myriotrochus and many species of Synapta and Chiridota it 

 is thin and transparent. The body wall consists of a single layer 

 of non-ciliated epithelial cells which carry a cuticle, a thick cutis 

 which consists of connective tissue and contains the calcareous 

 todies, a layer of circular muscles which is often interrupted in 

 the radii, five radial bands of longitudinal muscles (Fig 181), 

 each of which may be double, and finally the layer of peritoneal 

 epithelium lining the body cavity (Fig. 180). The cutis consists 

 of a ground substance containing branched cells and fibres. The 

 calcareous bodies of the cutis are minute in size and definite in 

 shape* : they have the form of anchors, wheels, rods, perforated 

 plates, stools, etc., and their shape and arrangement is of im- 

 portance for the determination of species. They are found in 

 the cutis of the tube-feet, ambulacral appendages, and tentacles, 

 as well as in the body wall. In a few forms, e.g. Psolus, Theelia, 

 the calcareous bodies of the dorsal side are large and plate-like 

 and appear like protective scales. In the Dendrochirotae in 

 which the anterior part of the body is invaginable, there are 

 at the base of the invaginable part five calcareous plates the 

 oral valves, which cover over the aperture when the proboscis 

 is withdrawn. Similar plates are sometimes found round the 

 anus. Both oral and anal plates may be radial or interradial 

 in position. 



Retractor muscles capable of retracting the anterior part of 

 the body are found in the Dendrochirotae and some other forms 

 (Molpadia, species of Chiridota and Synapta). They are muscular 

 bands detached from the longitudinal muscles at about the 

 middle of the body and inserted into the radial pieces of the 

 calcareous ring (Fig. 179). 



The calcareous ring consists of a circle of ten calcareous pieces 

 five radial and five interradial which surround the oeso- 

 phagus (Fig. 181, 28). It is placed in the outer wall of the 

 perioesophageal sinus (Fig. 179) on the oral side of the water- 

 vascular ring but aboral of the nerve ring. The interradial 

 pieces of the ring may be wanting or there may be more than 

 five interradial pieces. 



The water- vascular system consists of (1) a circular vessel 



* In some Holothurians they appear to change their shape with 

 advancing age, e.g. Stichopus Japonicus, Ann. Zool. Japonenses, 1, 1897, 

 p. 35. 



