272 PHYL UM ECHINODERMA. 



some others the body cavity serves as a brood-pouch. This 

 illustrates how the same result may be reached in a great 

 variety of ways. 



The calcareous plates of Holothurians are found as far 

 back as Carboniferous strata. 



As " trepang " or " beche-de-mer," the Holothurians of 

 the Pacific form an important article of commerce, being 

 regarded as a delicacy by the Chinese. 



Classification. 



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

 external tentacles, tube-feet, and ambulacral papillae, but the 

 tube-feet and papillae 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 Stichopus^ and Dendrochirotse, e.g. Cucumaria, 

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

 diidae with tentacles only, e.g. Molpadia\ the Pelagothuriidse 

 containing the free-swimming Pelagothuria. 



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

 the water-vascular system are the pinnate tentacles around the 

 mouth. One family, Synaptidae, 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" and bearing pinnules; the arms arise from a 

 or responding 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" 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 modem Comatulids, e.g. the rosy feather-star 

 ( Comatula or Antedon rosacea) 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 



