ECHINODERMATA 649 



gonad which adjoins it being displaced along their interradius to a 

 position not far from the mouth, and the other gonads lost. As has 

 been explained on p. 627, the madreporite usually becomes internal. 

 It is so in Holothuria. 



Owing to the presence in one interradius of the primary madre- 

 porite and the gonad, the body always possesses a rudimentary bi- 

 lateral symmetry. In many cases, as in Holothuria^ this symmetry 

 becomes conspicuous owing to the fact that the animal constantly 

 applies to the ground the three radii of the side opposite to the madre- 

 poric interradius, and this side becomes differentiated as "ventral" 

 from the "dorsal" side which contains the madreporic interradius 

 with the two radii which adjoin it. The differentiation consists in a 

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



Fig- 455- A diagram of a transverse section of a holothurian. d.b.v. dorsal 

 " blood vessel " ; d.i. dorsal interradius ;^. gonad ; int. 1,2, 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 inter- 

 radius; r.d.ra. right dorsal radius; r.ra. right radius; r.v.i. right ventral inter- 

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



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 but are withdrawn by means of retractor muscles, inserted into the 

 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 withdrawn by retractor muscles or by ampullae or by both. 



