CHAP. VIII ECHINODERMATA— SYSTEMATIC REVIEW 



285 



Systematic Review. 



CLASS I. Holothurioidea. 



The body is elongated along its principal axis ; it is cylindrical or vermiform. It 

 shows more or less distinct bilateral symmetry. The integument is soft or leathery, 

 and contains irregularly arranged, generally microscopically small, calcareous bodies. 

 The moutli lies at the oral (anterior) end of the principal axis of the body, and is 

 surrounded by feelers. The anus lies at the apical (posterior) end of the principal 

 axis. Ambulacral or tube-feet are either present or wanting. An external madre- 

 porite is usually not found. 



Ordee 1. Actinopoda. 



All the outer appendages of the water vascular system arise from the radial 

 canals, and take the form of feelers round the mouth 

 and of tube-feet (and ambulacral papilla) in other 

 parts of the body ; such feelers are always present, 

 the feet and papilla, however, may be wanting. 



Family 1. Aspidochirota. 



Tube -feet present. Mouth often more or less 

 ventral in position. Body usually shows distinct 

 flattening of the ventral surface. 18-30 peltate 

 tentacles. Tentacular ampullae well developed. 

 Stone canals often numerous. Retractor muscles 

 wanting. Respiratory trees present. Cuvier's 

 organs often present. Millleria, Holothuria, 

 Stichopus. 



i~ 



Family 2. Elasipoda. 



Tube-feet present. Mouth more or less ventral 

 in position. Body almost always distinctly flattened 

 on the ventral surface. 10, 15, or 20 tentacles, more 

 or less peltate in shape. Stone canal always single, 

 and not infrequently in direct communication with 

 the exterior through the integument. Retractor 

 muscles wanting. Respiratory trees wanting or 

 quite rudimentary. Cuvier's organs wanting. 

 Sub-fam. Psychropotidse : Psychropotes (Fig. 223), 

 Benthodytcs. Sub-fam. Deimatidae : Deinia, Pan- 

 nychia, Lactmogone. Sub-fam. Elpidiidse : Elpidia, 

 Kolga, Peniagone. 



Family 3. Pelagothnriidse. 



Tube-feet wanting. Mouth and anus terminal. „ „ ^ . , . 

 Til ,-i-,T,, n ,. Fig. 223.— Psychropotes longi- 

 Body cylindrical ; round the crown of tentacles it cauda (after Thdel). i, Oral ten- 

 widens out into a thin disc, the edge of which is tacle ; 2, mouth ; 3, 4, 8, aiubulacral 

 produced into long rays. 13-16 tentacles. Re- appendages of the (ventral) trivium ; 



tractor muscles wanting. Neither resijiratory trees, ^' ''""^ = "' "^^'"'^^ appendage with 



... , r^ ■ , its two posterior processes (7). 



nor ciliated organs, nor Cuvier s organs present. 



Calcareous bodies altogether wanting. Pelagic, swimming by means of the disc. 



Single genus and species : Pelagothuria natatrix (Figs. 224 and 225). 



