S88 ECHINODERMATA — CRINOIDEA chap. 



tropical species, the groove is supported by side-plates, these are 

 notched for the reception of the sacculi. 



Turning now to survey the group Crinoidea as a whole, lack 

 of space forces us to confine our attention mainly to the living 

 forms. These differ amongst themselves chiefly in the following 

 points: (1) the condition of the stem; (2) the structure of the 

 calyx ; (3) what is intimately connected with this, the method 

 of branching of the arms ; and (4) the length of the alimentary 

 canal. 



Condition of the Stem. — This is represented by a centro- 

 dorsal stump in Antedon and most of its allies, but in Actino- 

 tnetra it becomes a flat plate, and in some species in old age 

 all the cirri drop off In Uintacrinus and Marsupites (fossil 

 genera) there is no trace of cirri. In Pentacrinidae there 

 is a long stem, pentagonal in cross-section, in which alternate 

 ossicles carry whorls of cirri ; in Ehizocrinidae the stem consists 

 of compressed ossicles, elliptical in section, bearing cirri only 

 at the rooting tip, whilst in Syocrinus the stem is made up of 

 cylindrical ossicles, cirri being apparently absent. Finally, in 

 Holopus the stem is represented by an uncalcified leathery out- 

 growth from the calyx. 



Skeleton of Calyx and Arm. — In living Crinoidea, with the 

 doubtful exceptions of Holopus and Hyocrinus, the calyx is sup- 

 posed to be built up originally of four whorls of plates, viz. " infra- 

 basals," " basals," " radials," and " orals," the last named forming 

 the skeleton of the oral valves round the mouth. In the two 

 exceptions named there is no certain evidence of the existence of 

 infra-basals. In living forms the infra-basals coalesce with the 

 uppermost joint of the stem ; the basals remain large and 

 conspicuous, though they are fused into a ring in Ehizocrinidae, 

 Atelecrinus, and Thaumatocrinus, whilst in Pentacrinidae this 

 ring is nearly, and in Antedon and its allies completely, hidden 

 when the calyx is viewed from the outside. In Hyocrinus the 

 basals are represented by three ossicles. The lowest radials are 

 an important element in the patina in every case, but the 

 upper radials, the incipient portions of the arms, may be in- 

 corporated in the calyx {Pentacrinus, Antedon) or may be free 

 (Ehizocrinidae and Hyocrinus) ; in Metacrinus there are five to 

 eight radials in each column, all incorporated. 



The oral plates are very large in Hyocrinus, Holopus, and 



