n. plEchinoidea(Zoöl) The class Echinodermata which includes the sea urchins. They have a calcareous shell, usually more or less spheroidal or disk-shaped, composed of many united plates, and covered with movable spines. See SpatangoidClypeastroid.
Century Dictionary and Cyclopedia
EchinoideaA class of the phylum or subkingdom Echinodermata; the sea-urchins or sea-eggs. They have a rounded, depressed (not elongated) form, subspherical, cordiform, or discoid, inclosed in a test or shell composed of many calcareous plates closely and usually immovably connected, studded with tubercles and bearing movable spines, and perforated in some places for the emission of tube-feet; an oral and anal orifice always present, a convoluted intestine, a water-vascular system, a blood-vascular system, and sometimes respiratory as well as ambulatory appendages. The perforated plates are the ambulacra, alternating with imperforate interambulacral plates; there are usually five pairs of each. The anus is dorsal or superior, the mouth ventral or inferior; the latter in many forms has a complicated internal skeleton. The general arrangement of parts is radiate or actinomeric, with meridional divisions of parts; but bilaterality is recognizable in many adults, and perfectly expressed in the larval forms. The Echinoidea are divisible into Regularia, Desmosticha, or Endocyclica, containing the ordinary symmetriclly globose forms, as Cidaris, Echinus, and Echinometra; and the Irregularia, Petalosticha, or Exocyclica, containing the cake-urchins and heart-urchins, or the clypeastroids and spatangoids (respectively sometimes erected into the orders Clypeastrida and Spatangida); together with the Paleozoic echinoids, which in some systems constitute a third order, Palæchinoidea. Also Echinoida.
Webster's Revised Unabridged Dictionary
NL. See Echinus, and -oid
Echinoidea, a group of echinoderms, 61.
"The Biological Problem of To-day" by Oscar Hertwig
Echinoidea, 121, 122, 123.
"Elementary Zoology, Second Edition" by Vernon L. Kellogg