vin ECHINODERMATA BODY MUSCULATURE 471 



A. Holothurioidea. 



The dermomuscular tube consists everywhere of an outer circular 

 muscle layer, and of five radial longitudinal muscles (Figs. 371 and 

 383, pp. 451 and 477). 



The circular musculature lies immediately within the cuticle. It 

 is usually interrupted in the five radii, and thus consists of five longi- 

 tudinal interradial strips or bands of muscle, the fibres of which run 

 transversely. In the Pamd'mopoda (Syna-ptidw) alone, where there are 

 no radial canals of the water vascular system, the fibres run uninter- 

 ruptedly round the body. 



The longitudinal musculature consists of five strong muscles or 

 pairs of muscles traversing the body longitudinally along the radii. 

 These muscles thus cover, on the side of the body cavity, the radial 

 organs enumerated on p. 409. Anteriorly (at the oral pole) they are 

 inserted into the radial pieces of the calcareous ring, posteriorly (at 

 the apical pole) they end near the anus. 



In the Dendrochirotce, the longitudinal musculature is differentiated 

 in a peculiar manner. At the middle of the body, or somewhat in 

 front of it, the fibres of each of the five longitudinal muscles divide 

 into two bundles. One of these bundles is continued simply as a 

 longitudinal muscle along the body wall, while the other freely tra- 

 verses the body cavity, and is attached anteriorly to a radial piece of 

 the calcareous ring (Fig. 349, p. 404). The retractor muscles of the 

 oral region have in this way been derived by the splitting up of the 

 originally simple longitudinal muscles, and this specialisation became 

 more marked as the oral tentacles of the Dendrochirotce became more 

 and more highly developed, and required increasing protection. 

 Species are to be found in which the separation and branching off' of 

 retractors from the longitudinal muscles has not yet been perfected. 



Apart from the Dendrochirotce, retractors occur only in the genus 

 Molpadia, and in species of the genera Chirodota and Synapta. 



B. Eehinoidea. 



The longitudinal muscles of the Echinothuridce (Asthenosoma) have 

 the shape of semilunar leaves, the convex sides of which are directed 

 outwards, and are attached to the inner surface of the test ; the 

 concave edges, on the other hand, face the axis of the test (Fig. 380). 

 They are inserted into the test at the boundaries between the 

 ambulacra and the interambulacra, at the lateral edges of the ambu- 

 lacral plates. 



In each muscle lamella, the fibrous bands radiate fan-like (Fig. 380) from a 

 "centrum tendineum " on the inner edge of the lamella. The uppermost fibres are 

 attached to the radials, the lowermost to the outer side of the auriculae. Anasto- 

 moses between the fibres are not infrequent. 



