ix PHYLUM ECHINODERMATA 411 



mouth and five narrow ambulacral grooves lodging the tube-feet. 

 The larva has the form either of a bipinnaria or of a brachiolaria. 

 This class includes the Starfishes. 



ORDER 1. PHANEROZONIA. 



Asteroidea with large marginal ossicles. The dermal branchiae 

 are present only on the aboral surface. The ambulacral ossicles 

 not closely crowded. Pedicellarise sessile. 



ORDER 2. CRYFI-OZOXIA. 



Asteroidea with the marginal ossicles inconspicuous. Dermal 

 branchiae not restricted to the aboral, but often present on the 

 oral surface. Ambulacral ossicles crowded together. Pedicellariae 

 stalked or sessile. 



CLASS II. OPHIUROIDEA. 



Star-shaped free Echinoderms, with a central disc and five arms, 

 which are more sharply marked off from the disc than in the 

 Asteroidea and which cofitain no spacious prolongations of the 

 ccelome. There are distinct oral and aboral surfaces. The anus 

 is absent ; the mouth, as well as the madreporite, on the oral 

 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. 336 and 337). 



ORDER 1. LYSOPHIUR.E. 



Extinct Ophiuroids with ambulacral grooves. 

 Silurian and Devonian. 



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. 



