HOLOTHUROIDEA 



573 



more or less marked flattening of the ventral side and the loss, or 

 conversion into pointed sensory papillae, of the tube feet on the 

 dorsal side. The tube feet may be confined to the radii or may, as in 

 Holothuria, spread over the interradii, obliterating externally the 

 radial arrangement, though internally the radial structures retain 

 their position. 



The tentacles may be much branched (dendritic)^ provided with 

 lateral branches only {pinnate), or, as in Holothuria, furnished only 

 with a terminal circle of short branches, which themselves branch 

 {shield-shaped). Shield-shaped tentacles are retractile owing to the 

 presence of ampullae. Dendritic tentacles do not possess ampullae 

 but- are withdrawn by means of retractor muscles inserted into the 



r.d.ra. 



l.d.ra. 



Fig. 430. A diagram of a transverse section of a holothurian. d.b.v. "dorsal 

 blood vessel"; d.i. dorsal interradius; g. gonad; int. i, 2, 3, the three 

 stretches of the intestine; l.d.i. left dorsal interradius; l.d.ra. left dorsal 

 radius; l.ra. left radius; l.v.i. left ventral interradius; mes. mesentery; r.d.i. 

 right dorsal interradius; r.d.ra. right dorsal radius; r.ra. right radius; r.v.i. 

 right ventral interradius; v.b.v. "ventral blood vessel"; v.ra. ventral radius. 



radial pieces of the calcareous ring around the oesophagus (see below) , 

 which pull in the tentacular crown as a whole. Pinnate tentacles are 

 withdrawn by retractor muscles or by ampullae or by both. 



In the body wall, under the dermis, which contains minute ossicles 

 of a form characteristic of the species, there are transverse muscles 

 between the radii and longitudinal muscles under the radii. The 

 radii contain the same structures as in the Echinoidea. Only one tube 

 runs from the radial water vessel to each tube foot. 



The alimentary canal (Fig. 431) is slung to the body wall by a 

 mesentery. It runs (except in Synapta) an S -shaped course, looping 

 almost the whole length of the body — -backwards in the mid-dorsal 

 interradius, forwards in the left dorsal interradius, and finally back- 

 wards in the right ventral interradius to the anus. It starts as an 

 oesophagus^ enclosed in a calcareous ring of ten ossicles, five radial and 



