550 THE ANIMAL KINGDOM. 



vessels, or the trivium, and to the larval ventral surface, is distinctly flat- 

 tened as in many Aspidochirotae ; and the flattening may be carried so far 

 as to form a creeping sole, as in Psolus and Colochirus among Dendrochi- 

 rotae and all Elasipoda. The surface corresponding to the remaining two 

 ambulacral vessels or the bivium the larval dorsum is then more or less 

 convex. In Rhopalodina elegans the dorsal surface is extremely short so 

 that the mouth and anus are closely approximated, while the ventral sur- 

 face is large and convex. The body walls are in many Elasipoda pro- 

 duced into a distinct rim bordering the ventral surface, and in the family 

 Psychropotidae the dorsum is produced, as in the larva, anteriorly to the 

 mouth, which thus lies at some distance from the anterior extremity. The 

 body walls consist of a cuticle, an ectoderm, a connective tissue layer of 

 some thickness imbedding pigment and other cells, calcareous deposits 

 and bundles of nerve fibrils ; of a layer of circular or transverse muscles, and 

 five radial bands of longitudinal muscles placed internally to the radial 

 nerves and water-vascular vessels. The circular muscles are sometimes 

 interrupted in the radial lines, in Elasipoda, and except in this group, 

 the longitudinal muscles consist in each radius of a right and left band. 

 These bands are attached anteriorly to the radially placed pharyngeal 

 ossicles, and they each give off in Dendrochirotae retractor muscles for the 

 pharynx. The apical system of plates is not represented. The five cal- 

 careous plates in Millleria, &c., or the ten in Rhopalodina which surround 

 the anus have probably nothing to do with it. The oral system is repre- 

 sented by five calcareous valves which close over the mouth and tentacles 

 in Psolus ephippiger, or the young Psolimts brevis. !t is perhaps present 

 also in some larval forms. The calcareous 'deposits of the body are as a 

 rule represented only by scattered spicules as in most Elasipoda, by 

 wheels ( = rotulae), e.g. in Chirodota, by anchors with plates in Synapta or 

 variously shaped plates, all however small in size. Well developed cal- 

 careous plates are found in some instances, e. g. in Ocnus and Psolus ; and 

 in Echinocucumis and Echinosoma they carry spines. Overlapping plates 

 have been observed in a larval Holothuria and Cucumaria. Spicules are 

 commonly found in the walls of the tube feet, and in Elasipoda they occur 

 in various tissues, e. g. mesentery, walls of digestive tract, &c. The body 

 walls are lined by a ciliated epithelium which is continued over the mes- 

 entery. The coelome is large, and a special section of it, the oesophageal 

 sinus, surrounds the pharynx*. The pharyngeal ossicles are formed in the 

 outer wall of this sinus. They are sometimes absent (Embolus] or very 

 rudimentary. Among the Elasipoda, the Elpidiidae have five radially 

 placed spicules ; the Psychropotidae five plates (?), and the Deimatidae a 

 ring-shaped network. Typically there are five radially placed ossicles, 

 notched or perforated, by the radial nerves and water-vascular vessels, 

 alternating each with a single interradial ossicle, but the number of the 



