CRINOIDEA. 239 



and Psolus ephippiger, where the eggs and young are 

 attached to the back of the mother. In Cucumaria laevigata 

 there is an invaginated brood-pouch ; in Synapta vivipara 

 and others the body cavity serves as a brood-pouch. 



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. — 



1. Elasipoda : primitive deep-sea forms, bilaterally symmetrical, with 



tube-feet on the ventral surface only, and with papilla? on the 

 back. The stone canal often opens externally by a pore. There 

 are no respiratory trees or Cuvierian organs. 

 e.g. Kolga, Elpidia. 



2. Pedata : with well-developed tube-feet and papillse, usually with 



respiratory trees and Cuvierian organs. 



e.g. Holothuria, Cucumaria, Psolus. Pelagolhuria, a very 

 remarkable form, is pelagic and free-swimming, and wholly 

 without lime. 



3. Apoda : without radial canals, tube-feet, or respiratory trees. 



e.g. Synapta, a remarkable animal, especially apt to break in 



pieces ; tentacles pinnate ; hermaphrodite ; with beautiful 



calcareous anchors and plates in the skin. 



Semper has described a strange animal, Rhopalodina lageniformis , 



from the Congo coast. It is like a globular flask, with mouth and anus 



close together at the narrow end, with ten ambulacral areas. 



Class Crinoidea. Feather-Stars. 



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 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 

 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 — 



