00 PHYSIOLOGICAL SERIES. 



are unstriped. Five pairs of ligaments (indicated by black 

 bristles) spring from the actinostome and are attached to 

 the furcate outer ends of the radii ; they prevent the raising 

 of this end by the contraction of the interradial muscle. 



C. 11. Section of an Echiuothurid (Astlienosoma liystrix) showing 

 the arrangement of the muscles, by whose contraction the 

 alterations in form of the creature are produced. Two 

 of the five pairs of muscle-sheets are shown. They lie 

 in the boundary lines between the ambulacra and inter- 

 ambulacra, taking origin from the outer ends of the 

 ambulacral plates. Each muscle-sheet is of crescentic form 

 and composed of more or less isolated bundles. The latter 

 radiate from a central tendon to the test. The fibres are 

 smooth. 



P. & F. Sarasin, Ergebnisse Naturwiss. Forschungen auf 

 Ceylon, Bd. i. 1888, p. 93. 



C. 12. A Sea-Cucumber (Holotliuria tubulosa) laid open from the 

 dorsum to show the muscles of the body and cloaca. The 

 muscles of the body-wall consist of an outer circular and an 

 inner longitudinal layer. The latter is strongly developed 

 and is broken up into five radial bands, each divided down 

 the middle by a conspicuous furrow. Each band originates 

 from the calcareous ring around the oesophagus and extends 

 without interruption to the posterior extremity of the body. 

 A large number of fine branching strands of muscle radiate 

 outwards from the cloacal wall to the neighbouring parts of 

 the body-wall. They act as dilators of the cloaca. 



C. 13. Another specimen of ffolothuria tululosa, cut open to show 

 the muscles of the body-wall. 0. C. 56. Hunterian. 



C. 14. The dermo-muscular tube of a Gephyrean worm 

 (Sipuncuhts nudus} opened along the dorsal surface. 



There are three muscle-layers in the body-wall, whose 

 fibres (beginning with the outermost layer) have, respec- 

 tively, a circular, diagonal, and longitudinal direction. In 

 each layer the fibres are grouped together in regular par- 

 allel bundles. There are four retractor muscles of the intro- 



