136 ECHINODERMA. PELMATOZOA 



II. PELMATOZOA 



The Pelmatozoa, unlike the Eleutherozoa, are generally 

 sedentary, being attached to the sea-floor or some foreign 

 object by the aboral surface, usually by means of a jointed 

 stem ; in most cases the attachment is permanent, but it 

 may be temporary only. The group is essentially dis- 

 tinguished by the ciliated grooves which radiate from 

 the mouth ; the cilia produce a current of water which 

 carries small organisms to the upwardly directed mouth. 

 The Classes of the Pelmatozoa are: (1) Crinoidea, 

 (2) Cystidea, (3) Blastoidea, (4) Edrioasteroidea. 



CLASS I. CRINOIDEA 



The Crinoidea include the sea-lilies and feather-stars. 

 The skeleton consists of a stem, a calyx, and movable arms 

 given off from the margin of the calyx (fig. 55). 



The calyx is more or less globular, or cup- or basin- 

 shaped, and contains the digestive and other important 

 organs ; the mouth is either at or near the centre of the 

 ventral or oral surface, and the anus, which is excentric 

 and inter-radial, is also on the oral surface, and is usually 

 situated at the end of a tubular process. There is a groove 

 on the ventral surface of each arm, and these grooves — 

 the food-grooves — are continued over the oral surface to the 

 mouth ; they are lined with cilia, by the movements of 

 which food is conveyed to the mouth. There are generally 

 five arms, but each may branch repeatedly. Immediately 

 under the groove of each arm there is a radial nerve-cord ; 

 these cords unite to form larger trunks and ultimately join 

 as a ring round the mouth. Beneath the nerve of each arm 

 is a radial vessel of the water- vascular system, which is 

 continued over the oral surface and joins a ring round the 



