IX 



PHYLUM ECHINODERMATA 403 



surface. Except in one fossil order there are no ambulacral 

 grooves. The larva is a pluteus. This class includes the Sand- 

 stars and Brittle-stars (Figs. 341 and 342). 



ORDER 1. LYSOPHIUR.E. 



Extinct Ophiuroids with ambulacral grooves. 



Silurian and Devonian. 



i 



ORDER 2. STREPTOPHIUR^E. 



Ophiuroids in which the ambulacral ossicles articulate with one 

 another by simple ball-and-socket joints. 



ORDER 3. CLADOPHIUR^E. 



Ophiuroids in which the ambulacral ossicles articulate with one 

 another by means of hour-glass-shaped surfaces. The arms may 

 be branched. 



ORDER 4. ZYGOPHIUR^:. 



Ophiuroids in which the movement of the ambulacral ossicles on 

 one another is restricted by the presence of lateral processes and 

 pits. 



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. REGULARIA. 



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. 346). 



ORDER 3. SPATANGOIDEA. 



Heart-shaped Echinoidea^ with the mouth and anus excentric. . 

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

 (Fig. 345). 



D D 2 



