182 INVERTEBRATE ZOOLOGY 



Outline of Classification 



Phylum Echinoderma. — Triploblastie; coelomate; radially symmetrical; 

 calcareous plates in skin; water-vascular system; marine. 



A. Subphylum Eleutherozoa. — Echinoderms without stalk. 



I. Class Asteroidea. — A disc with five radiating arms; arms not sharply 

 set off from disc; ambulacral feet in grooves on oral surface. 



1. Order Forcipulata. — Marginal plates inconspicuous; stalked 

 pedicellariae; tube-feet with sucking discs. Asterias, Pisaster, 

 Evasterias, Leptastcrias, Orthasterias, Astrometis, Pycnopodia. 



2. Order Spinulosa. — Marginal plates inconspicuous; pedicellariae 

 wanting or at least not stalked; tube-feet with sucking discs. 

 Henricia, Solaster, Crossaster, Asterina. 



3. Order Phanerozonia. — Marginal plates large; pedicellariae usu- 

 ally sessile or in pits; tube-feet pointed or with discs. Ceramaster, 

 Mediaster, Dermasterias, Linckia, Astropecten, Luidia. Ctenodiscus. 



II. Class Ophiuroidea. — Arms slender, sharply set off from disc; no 

 ambulacral grooves. 



1. Order Ophiurae. — Arms unbranched. Ophioderma, Ophiura, 

 Ophionereis, Ophiothrix, Ophiopholis, Amphioplus. 



2. Order Euryalae. — Arms branched. Astrophyton, Gorgono- 

 cephalus. 



III. Class Echinoidea. — No arms; rigid test; globe or disc-shaped. 



1. Order Regularia. — Test nearly globular; mouth and anus in 

 dorsoventral axis; large spines; Aristotle's lantern present. 

 Arbacia . Strongylocentrotus, Toxopneustes. 



2. Order Clypeastroidea. — Test more or less flattened; anus at 

 margin of test; spines very small; Aristotle's lantern present. 

 Mellita; Clypeaster, Dendraster, Echina r achni us . 



3. Order Spatangoidea. — Test heart-shaped; mouth and anus 

 eccentric; no Aristotle's lantern. Spatangus, Lovenia. 



rV. Class Holothuroidea. — No continuous skeleton; plates small, 

 scattered in skin; elongate; cylindrical; mouth surrounded by a circle 

 of tentacles. 



1. Order Pedata. — With tube-feet. TJuione, Cucumaria, Psolus, 

 Stychopus. 



2. Order Apoda. — Without tube-feet. Synapta, Synaptula, Lepto- 

 synapta. 



B. Subphylum Pelmatozoa. — Stalked at least during part of life. 



I. Class Crinoidea. — Temporarily or permanently attached by a stalk; 

 five feathery arms radiating from a cuplike disc. Antedon, Penta- 

 crinus, Rhizocrinus, Metacrinus. 



II. Class Cystoidea. — Fixed, stalked, or sessile; varying number of 

 unbranched arms; wholly fossil. Lower Silurian to Carboniferous. 



III. Class Blastoidea. — Stalked; arms lacking; wholly fossil. Upper 

 Silurian to Carboniferous. 



rV. Class Edriasteroidea. — Theca of irregular plates; arms unbranched 

 and lying on theca; wholly fossil. Cambrian to Carboniferous. 



