66 



C(ELENTERATA AND 



< Astrophytidm. 

 Disk and arms with thick scaleless skin. Radial shields 



extend to centre of disk, forming elevated radial ribs. 



Arms branched many times. No arm-spines except at tip 



of bran chlets, where there 

 are microscopic booklets. 

 Arms folded ventrally. 

 Radial ribs yellow ; in- 

 terbrachial region brown 

 or red. Radial ribs with 

 short conical spines. In- 

 terbrachial region of disk 

 smooth or with short 

 spines. Ventral plates 

 replaced by integument. 

 Gorgonoce halus Agas- 



GORGONOCEPHALUS. 



SIZll. 



ECHINOIDEA. 



Body cylindrical, disk-shaped, without arms. Calcare- 

 ous, inflexible, composed of immovable plates. Apical 

 area with anus or destitute of same. Ovarian openings, 

 eye-spots and madreporic body around the apical area. 

 Locomotion by suckers and spines. Five double rows 

 of ambulacra! and five rows of inter-ambulacral plates. 

 The teeth are highly specialized into an apparatus called 

 the Aristotle's lantern. Pedicellariee pedunculated, tri- 

 fid. 



1. Echinoids. Body spherical with ambulacral zones 

 equal in length, unmodified from apical to oral region. 

 Aristotle's lantern. Development with pluteus. Ovaries 

 five. 



//. Clypeastroids. Body flat, disk-shaped ; ambula- 

 cral openings on the aboral surface, modified into five pairs 



