3fi 
Fig. 26.— ASTEROIDS. 
Fig. 26 : a, Pateaster matutina ; b, P. RhaefTeri ; c, P, NiagarenMS ; rf, P. Eucharis ; «. 
Onychaster flexilis, the rays folded. 
ECHINOIDEA. 
This, tne highest type of the Echinodermala (spiny skins), 
contains the true Sea urchins, or Sea-eggs, as they are often 
called. They consist of spheroidal, hollow bodies, usually covered 
with spines, whence their name, from Echinus, HeJgc-hog, in 
allusion to the spines. They are composed of distinct plates, 
arranged in regular geometrical order, often highly ornamented, 
and are always marked by five perforated areas, which radiate from 
one of the central openings, and which represent the rays of the 
star-fish, the arms of the Crinoid, or striated areas of the Blastoid. 
True Echinoids do not appear until the Hamilton period, and 
are not abundant until the Mesozoic age. 
