ix PHYLUM ECHINODERMATA :;7'J 



CLASS III. ECHINOIDEA. 



Free Echinoderms with globular, heart-shaped, or disc-shaped 

 body enclosed in a shell or corona of close-fitting, firmly united 

 calcareous plates. The mouth is nearly always polar : the anus 

 usually at the opposite (aboral) pole ; the madreporite is close to 

 the latter. There are no ambulacral grooves ; but the surface is 

 divided into alternating ambulacral and inter-ambulacra! zones or 

 areas, which usually run from pole to pole. The larva is a Pluteus. 

 This class includes the Sea-urchins, with the Heart-urchins and 

 Cake-urchins. 



ORDER 1. PALJEO-ECHINOIDEA. 



Fossil Echinoidea in which the number of rows of plates in the 

 corona is variable, and in which the plates overlap one another. 



ORDER 2. REGULARIA. 



Echinoidea with globular corona containing twenty meridional 

 rows of plates. Mouth and anus polar. A lantern of Aristotle is 

 present. This order includes the Sea-urchins. 



ORDER 3. CLYPEASTRIDEA. 



Echinoidea with more or less flattened corona, with the mouth 

 central, the anus excentric. A lantern of Aristotle is present. 

 This order includes the Cake-urchins. 



ORDER 4. SPATANGOIDEA. 



Heart-shaped Echinoidea with the mouth and anus excentric. 

 No lantern of Aristotle. This order includes the Heart-urchins. 



CLASS IV. HOLOTHUROIDEA. 



Free Echinoderms with elongated, cylindrical or five-sided body, 

 having the mouth and anus at opposite extremities. The body- 

 wall is usually only supported by scattered ossicles or spicules. 

 There is no external opening to the madreporic canal (except in 

 some Elasipodif}. The surface usually exhibits five ambulacral 

 areas ; but these may be absent. There is a circlet of large oral 

 tentacles. The larva is an Auricular ia. This class includes the 

 Sea-cucumbers- and Beche-de-mer. 



ORDER 1. ELASIPODA. 



Holothuroidea with well-marked bilateral symmetry, with tube- 

 feet on the ventral surface (which is flattened) and papillae on the 

 dorsal. Confined to the deep sea. 



ORDER 2. PEDATA. 



Holothuroidea with tube-feet either in longitudinal rows or 

 scattered irregularly over the surface. 



