CRINOIDEA 619 



CLASS 1. CRINOIDEA. 



Sea-lilies. Feather-stars. Echinoderms which, either throughout life 

 or during their youth alone, are attached by a stalk to the sea bottom. 

 The body of the typical crinoid is made up of three parts, the disc or 

 calyx, the arms, and the stalk. The calyx is more or less cup-shaped and 

 forms the central mass of the body. Its oral surface, with the mouth, 

 is directed upwards; near the mouth and raised above the surface in an 

 interradius is the anus. The aboral side is directed downwards and is 

 attached to the stalk when this is present. In some crinoids the stalk 

 bears whorls of jointed appendages called cirri. In most living crinoids 

 the stalk is wanting in the adult animal. 



The arms are either five or ten in number, and often branch di- 

 chotomously. They bear small alternating branches called pinnules which 

 give them a feathery appearance. Five ambulacral grooves radiate from 

 the mouth to the arms, and pass along their oral surface, branching with 

 them, extending to the tips of the pinnules. These grooves are lined 

 with cilia, which create a current running towards the mouth, to which 

 they aid in bringing food. Ambulacral appendages are present along 

 the edges of the grooves, which are tactile, respiratory, and excretory in 

 function. The general surface of the body is not ciliated. No special 

 sense organs are present. 



Internal Organs. The calcareous skeleton of crinoids is extensively 

 developed. The aboral wall of the calyx contains two groups of polyg- 

 onal plates which fit together closely and are often fused; these are the 

 basals (Fig. 968,1) and the radials (2). The former are at the end of 

 the stalk, occupying interradial areas; they are normally five in number, 

 but may be reduced by fusion, or increased to ten by the addition of five 

 infrabasals in the radial areas between the basals and the stalk. 



The radials are in the radial areas and lie above the basals, one or 

 more rows of them being present ; interradials are present in some species. 

 The oral body wall in Holopus and other primitive crinoids contains five 

 large oral plates; in most crinoids, however, a number of small plates is 

 here present, or there may be no plates at all. A row of large circular 

 plates called brachials is present in the arm, which join the radials. 



The stalk contains a succession of disc-like plates placed one on top 

 of another; the cirri are similarly jointed. 



The body cavity occupies the calyx, and extends through the arms to 

 the tips of the pinnules and through the stalk and cirri. In the calyx 

 it contains a network of connective tissue fibers which are often calcified. 



* See "Report on the Crinoidea," by F. H. Carpenter, Reports of the Challenger, 

 Vol. 26, 1888. 



