430 ECHINODERMATA ELEUTHEROZOA chap. 



decipher the past history of life from the fossil record, it is 

 necessary to have an exact and detailed knowledge of Echinoderm 

 skeletons and their relation to the soft parts. Lastly, the 

 internal organisation of Echinoderms throws valuahle light on 

 the origin of the complicated systems of organs found in the 

 higher animals. 



Eohinodermata are divided into two great sub-phyla, which 

 must have very early diverged from one another. These are : — 



(1) Eleutherozoa, (2) Pelmatozoa.^ 



The sub-phyium Pelmatozoa, to which the living Feather- 

 stars (Orinoidea) and tlie majority of the known fossil species 

 belong, is characterised by the possession of a fixing organ placed 

 in the centre of the surface opposite the mouth — the aboral 

 surface as it is called. Ordinarily this organ takes on the 

 form of a jointed stalk, but in most modern species it is a little 

 knob with a tuft of rooting processes, termed cirri. In the 

 other sub-phylum, the Eleutherozoa, no such organ is found, 

 and the animals wander about freely during their adult life, 

 though for a brief period of their larval existence they may be 

 fixed by a stalk -like protuberance arising from the oral 

 surface. 



SUB-PHYLUM I. ELEUTHEROZOA 



The Eleutherozoa are divided into four main classes, between 

 which no intermediate forms are found amongst the living species, 

 though intermediate types have been found fossil. 



The four classes into which the Eleutherozoa are divided 

 are defined as follows : — 



(1) Aster oidea (Starfish). — " Star "-shaped or pentagonal 

 Eleutherozoa with five or more triangular arms, not sharply 

 marked off from the central disc. The mouth is in the centre 

 of one surface, called from this circumstance the "oral " : the anus 

 is in the centre of the opposite surface, termed the " aboral." 

 From the mouth a groove runs out on the under surface of each 



^ This classification is substantially that suggested by Jeffrey Bell, Catalogue 

 of Brilish Echinodcr'ins in the British Museum, 1892, except that Bell separates 

 Holothuroidea from all others. Reasons will be given later for regarding Holo- 

 thuroidea as modified Echinoidea. 



