620 ECHINODERMATA 



The body cavity contains a blood fluid mixed with sea water, which is 

 admitted through ciliated pores in the oral wall of the calyx. 



The mouth opens into the tubular digestive canal, which runs once 

 round the main axis of the body cavity and then opens to the outside 

 through the anus. No digestive glands are present, but certain inter- 

 radial diverticula of the intestine in Antedon may have a glandular 

 function. 



The ambulacral system consists of a ring canal around the mouth and 

 five radial canals which branch with the arms and are continued in the 

 pinnules. The ambulacral appendages along the edges of the ambulacral 

 groove are without terminal suckers and ampullae. A number of stone 

 canals, which lack calcareous bodies in their walls, leave the ring canal and 

 end free in the body cavity. Five groups of ambulacral tentacles also 

 spring from the ring canal and surround the mouth. 



The superficial oral nervous system, consisting of an oral nerve ring 

 and a radial nerve in each arm, is situated just beneath the outer epi- 

 thelium; an apical system is also present, consisting of a nerve capsule in 

 the aboral calcareous plates and large radial nerves which run through 

 the brachials into the arms. The reproductive organs, the ovaries or 

 testes, extend throughout the arms and into the pinnules, where they 

 become ripe, and through minute pores of which the ova or sperm are 

 'extruded. The five gonads are usually joined with an organ called the 

 genital stolon, which lies in the axis of the calyx and is homologous to 

 the 'axial organ of other echinoderms. The sexes are separate. 



Distribution and Habits. The stalked crinoids are deep-sea forms, 

 and most numerous in East Indian waters, but are also found in nearly 

 all oceans; they constitute about one-fifth of living crinoid species. The 

 Antedonidae and Comasteridae are not stalked as adults, and are also 

 commonest in East Indian waters, varying from the shallow waters along 

 the shores to great depths, but are also found, and often plentifully, in 

 almost all seas; they do not occur, however, in the shallow waters of 

 New England. The stalked forms may be rooted to the bottom, but it is 

 likely that the stem is occasionally broken accidentally or perhaps by the 

 animal itself and that it then moves about by means of the arms. 

 Isocrinus is perhaps never permanently rooted, but anchors itself tempo- 

 rarily by its cirri, and possibly moves about at will. The Antedonidae 

 and Comasteridae are found among rocks and coral colonies and move 

 about slowly, swimming by means of their arms. 



History. Antedon, which has always been the best known crinoid, 

 was usually classed with starfishes by the earlier zoologists. J. S. Miller, 

 in 1821, first organized the group under the name Crinoidea, and E. 

 Forbes, in 1841, raised it to the dignity of an independent class. 



