IX PHYLUM ECHINODERMATA 413 



Order 2. — Pedata. 



Holothnroidea with tube-fuet either in kmgitudinal rows or 

 scattered irregularly over the surface. 



Order 3. — Apoda. 



Holothuroidea devoid of tube-feet and of radial ambulacral 

 vessels. 



SUB-PHYLUM II.— PELMATOZOA. 



Echinodermata which are usually fixed at the base, and usually 

 supported on a stalk composed of a row or rows of ossicles 

 (Fig. 342) : the mouth on the free surface, near or in the centre, and 

 having extending out from it on the oral surface a radially arranged 

 system of narrow, ciliated ambulacral grooves, having the function 

 of food-grooves, which may run between the plates of the theca, 

 on the surface of the theca, or along the oral surfaces of a system 

 of radial processes or arms given off from it. The tube-feet of 

 other Echinoderms, when represented, take the form of small, 

 tubular, strongly ciliated appendages (tentacles) without suckers : 

 the anus usually on the oral surface. 



CLASS I.— CRINOIDEA. 



Mostly fixed, stalked Pelmatozoa in which there is a theca 

 comprising five regularly arranged radial and five basal plates, 

 giving off five, usually branched, jointed processes or arms ; with 

 food-grooves radiating out from the mouth along the oral surfaces 

 of the arms, and extending along their branches : the central parts 

 of the ambulacral, nervous, and reproductive systems, and of the 

 coelome lodged in the theca, send extensions through the arms. 



This class comprises, together with many extinct forms, the 

 only living Pelmatozoa. 



Sub-Class I.^ — Monocyclica. 



Crinoidea in which the base of the theca comprises basals 

 only. 



Sub-Class II. — Dicyclica. 

 Crinoidea in which the base comprises basals and infra-basals. 



CLASS II.— CYSTOIDEA. 



Fixed, stalked, or sessile Pelmatoza, with the plates of the theca sometimes 

 irregular, sometimes arranged in a regular radial system, with food-grooves 

 extending for a longer or shorter distance over the surface of the theca, some- 

 times on special plates lying above those of the latter, their terminal parts 



