258 PHYLUM ECHINODERMA. 



Classification.- 



Order i. Actinopoda. The radial water vessels are associated with 

 external tentacles, tube-feet, and ambulacral papilla.-, but the 

 tube-feet and papilla? may be absent. There are several 

 families, e.g. the deep-sea Elasipoda, markedly bilateral, almost 

 always flattened ventrally, often with an external pore for the 

 stone canal, e.g. Elpidia and Kolga ; the Aspidochirotae, e.g. 

 Holothuria and Stichopns, and Dendrochirotae, e.g. Ci(ci//naria, 

 Thyone, Fsohis, with tube-feet as well as tentacles ; the Molpa- 

 diidre with tentacles only, e.g. Molpadia. 



Order 2. Paractinopoda or Apoda. The only external outgrowths of 

 the water- vascular system are the pinnate tentacles around the 

 mouth. One family, Synaptidre, e.g. Synapta and Chiridota. 

 There are no tube-feet or respiratory trees or Cuvierian organs. 

 The calcareous bodies are usually beautiful anchors and plates. 

 Many are hermaphrodite. 



Class CRINOIDEA. Feather-stars. 



Usually stalked forms, with five jointed^ often branched 

 arms (" brachia "), growing out from a central cup or 

 " theca" atid bearing pinnules ; the arms arise from a 

 corresponding number of thecal plates or " radials" below 

 which there is a circlet of alternating " basals" often with 

 " infra-basals " alternating again with them; below the 

 " basals" or "infra-basals' 1 ' there is usually a jointed stem 

 anchored to the substratum by "cirri." 



The feather-stars or sea-lilies differ from other Echino- 

 derms in being fixed permanently or temporarily by a jointed 

 stalk. The modern Comatulids, e.g. the rosy feather-star 

 (Comatula or Antedon rosaced) leave their stalk at a certain 

 stage in life ; but the other Crinoids, e.g. Pentacrinus, are 

 permanently stalked, like almost all the extinct stone-lilies 

 or encrinites, once so abundant. Most of them live in deep 

 water, and many in the great abysses. An anchorage is 

 found on rocks and stones, or in the soft mud, and great 

 numbers grow together a bed of sea-lilies. The free 

 Comatulids swim gracefully by bending and straightening 

 their arms, and they have grappling " cirri " on the aboral 

 side, where the relinquished stalk was attached. By these 

 cirri they moor themselves temporarily. Small organisms 

 Diatoms, Protozoa, minute Crustaceans are wafted down 

 ciliated grooves on the arms to the central mouth, which is 

 of course on the upturned surface. Some members of 



