CHAPTER XV 
ECHINODERMATA 
Asteroidea—Asterias, Solaster, Brisinga. 
Ophiuroidea— Astrophyton, Ophiopholis. 
Crinoidea—Comatula, Pentacrinus. 
Regulares—Zchinus, Toxopneustes. 
Clypeastroidea—Clypeaster, Rotula. 
Spatangoidea—Spatangus, Brissus. 
( Actinopoda—Holothuria, Cucumaria, Deima. 
( Paractinopoda—Synapta, Ohirodota. 
Echinodermata Bebencides 
Holothuroidea 
CHARACTERISTICS.— Animals with a primitive bilateral symmetry, 
which is in the adult replaced by a more or less regular radial 
symmetry, usually pentamerous. The skin 1s hardened by 
calcareous deposits, which 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 
Jrom the rest to form the water-vascular system, which ws 
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 excluswely marine. 
The Echinodermata are divided into five classes : 
I. ASTEROIDEA. 
II. OPHIUROIDEA. 
III. CriInompEA. 
IV. EcHINOIDEA. 
V. HOLOTHUROIDEA. 
