404 ZOOLOGY SECT. 



CLASS IV.-HOLOTHUROIDEA, 



Free Echinoderms with elongated, cylindrical or five-sided body, 

 having the mouth and anus at opposite extremities. The body- 

 wall is usually only supported by scattered ossicles or spicules. 

 There is no external opening to the madreporic canal (except in 

 some Elasipoda). The surface usually exhibits five ambulacral 

 areas : but these may be absent. There is a circlet of large oral 

 tentacles. The larva is an auricular ia. This class includes the 

 Sea-cucumbers and " Beche-de-mer." 



ORDER 1. ELASIPODA. 



Holothuroidea with well-marked bilateral symmetry, with tube- 

 feet on the ventral surface (which is flattened) and papillae on the 

 dorsal. Confined to the deep sea. 



ORDER 2. PEDATA. 



Holothuroidea with tube-feet either in longitudinal rows or 

 scattered irregularly over the surface. 



ORDER 3. APODA. 



Holothuroidea devoid of tube-feet and of radial ambulacral 

 vessels. 



SUB-PHYLUM 11,-PELMATOZOA. 



Echinodermata which are usually fixed at the base, and usually 

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

 (Fig. 347) : 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.-CBINOIDEA. 



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. 



