CHAPTEE XV 



ECHINODERMATA 



Eohinodermata 



Asteroidea — Asterias, Solaster, Brisinga. 

 Ophiuroidea — Astrophyton, OphiophoUs. 

 Crinoidea — Comatula, Pentacrhms. 



I Eegulares — JEchinus, Toxopneustes. 

 Echinoidea . -j Clypeastroidea— CTjfpcasier, Jtotula. 



(_Spataiigoidea — Spatangus, Brissus. 



tr 1 4.T, -J f Actinopoda — Eolothuria, Cucumaria, Deima. 

 Holothuroidea a _ , . , „ „, . 



( Paraotinopoda — Synapta, Omrodota. 



Chaeacteeistics. — Animals with a primitive bilateral symmetry, 

 which is in the adult replaced by a more or less regular radial 

 symmetry, usually pentamerous. The shin is hardened by 

 calcareous deposits, vjhich may take the form of scattered 

 spicules or of plates which build up an almost complete shell, 

 but in all cases they are mesodermic structures. A well- 

 developed coelom is present, and part of it becomes cut off 

 from the rest to form the tvater-vascular system, which is 

 both locomotor and respiratory in function. The five radial 

 vessels of this system correspond with five areas, the '' ambu- 

 lacra " ; the angles between them form the " interambulacra.'' 

 The alimentary canal usually opens to the exterior at both 

 ends, but an anus may be absent. The sexes are usually 

 distinct, and developement is nearly always associated with a 

 metamorphosis. They are exclusively marine. 

 The Echinodermata are divided into five classes : 



I. Asteroidea. 

 II. Ophiueoidea. 



III. Ceinoidea. 



IV. Echinoidea. 



V. HOLOTHUEOIDEA. 



