412 ZOOLOGY sect. 



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-ambulacral 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. — Kegularia. 



Echinoidea with globular corona containing, in most cases, 

 twenty meridional rows of plates. Mouth and anus polar. A 

 lantern of Aristotle is present. This order includes the Sea- 

 urchins. 



Order 2.— 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 (Fig. 341). 



Order 3. — Spatangoidea. 



Heart-shaped Echinoidea with the mouth and anus excentric. 

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

 (Fig. 340). 



CLASS IV.— HOLOTHTIROIDEA. 



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 Elasipoda). The surface usually exhibits five ambulacral 

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

 tentacles. The larva is an auricularia. 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. 



