380 ZOOLOGY SECT. 



ORDER 3. APODA. 



Holothuroidea devoid of tube-feet and of radial ambulacral 

 vessels. 



CLASS V. CRINOIDEA. 



Temporarily or permanently stalked Echinoderms with star- 

 shaped body, consisting of a central disc and a series of five 

 bifurcate or more complexly branched arms, bordered with pin- 

 nules. There are distinct dorsal and ventral surfaces ; the latter 

 bearing the mouth and anus, and the inner ends of a series of 

 narrow ambulacral grooves. This class comprises the Feather- 

 stars and Sea-lilies. 



ORDER 1. PAL^OCRINOIDEA. 



Stalked Crinoidea in which the disc is large as compared with 

 the arms, a number of inter-radial plates being present and often 

 united with the disc. The ventral surface usually concealed by a 

 " vault " of calcareous plates. 



Extinct, palaeozoic. 



ORDER 2. NEO-CRINOIDEA. 



Stalked or free Crinoidea in which the disc is small as com- 

 pared with the arms, and in which inter-radials, when present, do 

 not combine with the plates of the disc. There is no " vault " 

 covering the ventral surface. 



Comprising all the living forms, together with several extinct 

 mesozoic families. 



CLASS VI. CYSTOIDEA. 



Fossil Echinoderms with globular body, sometimes sessile, sometimes stalked, 

 enclosed in usually irregular, polygonal plates. Mouth central ; five radiating 

 ambulacral grooves. Palaeozoic. 



CLASS VII. BLASTOIDEA. 



Fossil Echinoderms with ovate stalked body, central mouth, and five ambu- 

 lacral areas. Palaeozoic. 



Systematic Position of the Examples. 



Asterias nibens is a species of the genus Asterias, which, with 

 several others, constitutes the family Asteriidce of the order 

 Cryptozonia. The family Asteriidcv is characterised among the 

 families of the Cryptozonia by the following distinctive features : 

 The ossicles of the dorsal surface are small, unequal, reticulate 

 plates, bearing isolated or grouped spinelets (paxillse). The margin 

 of the actinostome is defined by the ambulacral plates. The 

 pedicellarise are of two forms, forceps-like and scissors-like. The 



