310 PHYLUM ECHINODERMA 



very brightly coloured. The larva is, in most cases, what 

 is known as an Auricularia. Sometimes, however, the 

 larval stage is skipped, as in Cucumaria crocea and Psolus 

 ephippifer^ where the eggs and young are attached to the 

 back of the mother. In C. curata the eggs and young are 

 sheltered on the ventral surface ; in C. parva in a shallow 

 ventral insinking ; in C. Icevigata there is an invaginated 

 ventral brood-pouch ; in Chiridota contorta the young are 

 sheltered in the genital tubules ; in Synapta vivipara and 

 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 papilla, 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. FApidia and Kolga ; the Aspidochirota), e.g. 

 Holothuria and Stichopus, and Dendrochirota?, e.g. Cucumaria. 

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

 diidae with tentacles only, e.g. Molpadia ; the Pelagothuriidae 

 containing the free-swimming Pelagothuria. 



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

 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 arm.s arise from a 

 corresponding number of thecal plates or " radials,'' below 

 which there is a circlet of alternating " basals,^' often with 

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

 " basals " or " infra-hasals " there is usually a jointed stem 

 anchored to the substratum by ' cirri. ^^ 



