ECHINODERMA TA S Y STEM A TIG RE VIE W 



295 



Fam. 3. Leskiidse : Paheostoma (extant). Fani. 4. Pourtalesiidse : Pourtalesia 

 (Fig. 238), Spafa/jocystis, Uchiiiocrepis (extant).-^ 



Sa** 



Fig. 238.— Pourtalesia Jeffreys!, from the .side (after Loven). The smaller tubercles are not 

 depicted, op, Apex ; os, oral jjole ; en, anal region. The niimbers are explained in the text, in the 

 section on the perisomatic skeleton of the Echinoidea, p. 342. 



CLASS III. Asteroidea (Stelleridea), Star-fish. 



Ecliinodermata, with body flattened in the direction of the principal axis, the 

 radii being produced laterally into longer or shorter arms. The arms are usually five 

 in number, but their number may be increased to forty or more. They are not dis- 

 tinctly marked off from the central part of the body (the disc) ; and besides the radial 

 blood vessels, nerves, and ambulacral vessels, diverticula of the intestine and con- 

 tinuations of the genital organs run into the coelomic cavities of the arms. The 

 body is usually covered with calcareous plates, but is flexible. The calcareous 

 plates carry spines, and often pedicellariiE as well. Along each arm runs a ventral 

 furrow, within which there is a longitudinal row of paired ambulacral plates. The 

 consecutive pairs are movably articulated with one another. Besides these, there 

 are, on the arms, adambulacral, inframarginal, supramarginal, and dorsal plates. 

 The ambulacral gi'ooves run from the central mouth on to the arms, and along these 

 on their oral (ventral) side, below the ambulacral plates, to their tips. The tube- 

 feet (tentacles) rise from the base of this groove, to which they are limited. Anus 

 apical {i.e. in the centre of the upper side), rarely wanting. Madreporite also on the 

 apical side of the disc. The sexes are separate. Development is in most cases with 

 metamorphosis (free-swimming pelagic larvae) ; when the brood is protected develop- 

 ment is direct. 



Sub-Class 1. Palaeasteroidea. 



Palaeozoic Asteroidea, in which the ambulacral j^lates in tlie two longitudinal 

 rows in each arm, at least in the middle of the arm, are arranged alternately (not 

 opposite or in pairs). Aspidosoma, Palwaster, Pahcocoiiia, etc. (all Palaeozoic forms). 



^ The classification of the Echinoidea here given is after Martin Duncan, A Remsion 

 of the Genera and (rreat Groups of the Echinoidea. Loudon, 1889. 



