14 



ECHINODERMS OF THE BRITISH ISLES 



it has two articulating surfaces at its distal end, from each of 

 which a series of plates arise, which develop into the arm-joints. 

 Close to one of the radials lies a small plate, the anal jilate, which 

 later on disappears in most recent forms. With the growth of 

 the radials the orals and basals are gradually separated from 

 each other. The orals remain at the mouth-edge and ultimately 

 disappear in most Comatulids. The basals undergo a remarkable 

 metamorphosis ; gradually they disappear from the surface of 

 the calyx and are transformed within the calyx into a small plate, 

 the rosette, which lies as a cover over the cavity of the centro- 



dorsal. This latter is formed by the 

 uppermost stalk joint, which grows 

 very wide and gradually conceals 

 the radials. On the centro-dorsal the 

 cirri develop ; when a couple of whorls 

 of cirri has appeared, the young sea- 

 lil}^ detaches itself from the stalk and 

 then attaches itseK to the sea-bottom 

 by means of the cirri, while the stalk 

 falls to pieces. The fixed stage of the 

 Comatulids is designated the Penta- 

 crinoid stage (cf. Figs. 16, 17, 19, 25). 

 The development of the stalked 

 Crinoids is unknown. 



The Comatulids, as a rule, live 

 attached, by means of their cirri, to 

 the sea-bottom, or to other organisms, 

 moving their arms only feebly ; but 

 they are able to relax their hold and 

 swim freely, moving their arms alter- 

 natingly up and down (Fig. 4). Such 

 swimming feather-stars afford an exceedingly beautiful sight ; 

 the feather-stars, with their often very vivid colours, are, 

 indeed, among the most beautiful of marine animals. Among 

 fossil Crinoids a few forms are known which have led a perfectly 

 free, pelagic life ; no such forms are known to exist in recent times. 

 The food of the sea-lilies consists mainly of plankton organisms, 

 which are caught by means of the tentacles and carried to the 

 mouth by the cilia in the ambulacral grooves, which send 

 a constant current of water towards the mouth. Probably 

 also the oral pinnules, in some cases, help directly in catching the 

 food. Besides plankton organisms, detritus also appears to form 

 an important part of the food of sea -lilies. No other animals are 



Fig. 4. — Sketch of swimming 

 Feather - star {Antedon 

 bifida). Nat. size. 

 (From H. C. Chadwick.) 



