Cystoidea, Pelmatozoans with saclike body enclosed in an irregular box. The arms are poorly developed. The mouth is central. The primitive members of the class are spherical. Some genera tend to the Crinoid type; others resemble the echinoids. They appear in the rocks of the Upper Cambrian Age being the earliest group of echinoderms. They disap peared at the close of the Paleozoic.
Blastoidea, a small, extinct specialized group derived from the cystoids, which became extinct at the close of the Palaeozoic age.
Consult Lankester, E. R., 'Treatise on Zoology' (Part HI, London 1900), and articles on the several classes. Consult also Clark, W. B., (The Mesozoic Echinodermata of the United States' (In Bulletin of the United States Geological Survey, No. 97, Washington 1893) ; Ludwig,
ECHINOIDEA, or SEA URCHINS, a class of Echinodermata with a more or less spheroidal or discoidal body en closed in a continuous test or shell composed Polygonal pieces of various shapes accurately fitted together and arranged in radial rows..
Alternating ambulacral and interambulacral areas, usually repeated five times, may be recog nized. The former exhibit rows of perforations for the tube feet, generally reaching to the aboral pole. The skeletal plates support movable spines which sometimes attain a great size and are used in defense and to a certain degree in locomotion. In addition, the integu ment bears the remarkable structures known as pedicellaria, which are three-jawed pincers borne on movable stalks, which by continually snapping and bending carry away particles of fatal matter and secure cleanliness. The be: ginning of the digestive canal is frequently provided with a complicated masticatory ap paratus known as Aristotle's lantern, and the canal itself is tubular and looped, usually not pouched. The gonads are interradial with out, lets to the •exterior near the aboral pole. Development may be direct or through the larval form called pluteus, which is provided in its later stages with long processes supported by movable spicules. The number of known Echinoidea is 'very great, many of them being ,fossil. There are two sub-classes: Regularsa, form regular, spheroid; mouth and anus at opposite poles of the central axis, and the latter usually surrounded by small skeletal plates; in only a few cases are external gills present. Here belongs the common sea-urchin (q.v.). Irregularia, form markedly bilateral; anus al ways displaced from the apical pole into the posterior interradius and the mouth frequently in the opposite direction, in which case the form is sometimes heart-shaped. Examples are the sand-dollar and heart urchin (q.v.).