250 



PHYLUM ECHINODERMATA. 



(Dendrochirotae). The tentacles and the part of the body 

 carrying them are retractile. 



The ambulacra! feet terminate in a suctorial disc which is 

 provided with a perforated calcareous plate, while the ambula- 

 tory papillae have pointed ends and the calcareous plates are 

 reduced or absent. In the Elasipodidae calcareous plates are 

 only exceptionally present even on the feet. No sharp line can 

 be drawn between these two kinds of appendages ; and it is 

 often impossible to say whether we have to do with the one or 

 the other. The ambulacral feet are essentially locomotory, 

 while the papillae are respiratory and sensory. The distribution 

 of these structures varies considerably even in the same genus. 

 They may be arranged in radial rows, which may be single, 

 double or multiple ; or they may be scattered on radii and 



FIG. 178. Calcareous bodies from the integument of Holothurians. a Calcareous wheels 

 of ChirikOta ; h anchor with supporting plate of Synapta ; c stool-like body; d plate of 

 Holothtiria impatiens : e hooks of Chiridota. 



interradii alike. If there is a ventral sole, the three ventral radii 

 are provided with feet and the dorsal radii with papillae, scat- 

 tered or in rows. Sometimes the feet are absent from the 

 median row of the sole (some Elasipodidae, Fig. 184), and in 

 Psolus the dorsal surface is altogether without ambulacral 

 appendages. In the Molpadiidae, Synaptidae and Pelago- 

 thuria both feet and papillae are absent, the tentacles being the 

 only representative of ambulacral appendages. 



There is a dermo- muscular body wall, which contains isolated 

 calcareous spicules of various form (Fig. 178). Calcareous 

 plates such as are found in other Echinoderms are feebly if at 

 all developed and no representatives of the oral and apical 

 systems of plates are found at any stage of life. The skin has a 

 leathery consistency and may be covered with warts and ridges. 



