spatangoidaThe Spatangidæ, in a broad sense, as an order of petalostichous sea-urchins: synonymous in some uses with Petalosticha, but usually restricted to exclude the clypeastroids or flat sea-urchins: then also called Spatangida and Spatangina. The forms are numerous; most of them fall in the family Spatangidæ as usually limited. from which the Cassidulidæ are distinguished by the absence of semitæ and other approaches to the regular sea-urchins. The form of the spatangoida is various, and only a part of them have a cordate figure. Some are quite elongate, and may even bear a sort of beak or rostrum, as in the genus Pourtalesia. The tendency is away from radiism and toward a sort of bilateral symmetry, as evidenced by the disposition of five ambulacra in two groups, an anterior trivium—under the odd ambulacrum of which is the mouth—and a posterior bivium, in relation with which is the anus. The odd anterior ambulacrum often aborts, leaving apparently but four ambulacra on the upper surface; in other cases it is disproportionally enlarged. The ambulacra are always petaloid; semitæ are not recognized outside this group, and occur nearly throughout it (but not in Cassidulidæ and the fossil Dysasteridæ); the spines are very variable, and few or many, but always slender or fine, sometimes like hairs of great length. The genital and ocular plates are centric; there are no Polian vesicles, and four kinds of pedicels or tube feet occur, of which the semital are always different from the two or three kinds of ambulacral feet. See cuts under Ananchytes, Echinocardium, petalostichous, semita, and Spatangus.
Chambers's Twentieth Century Dictionary
ns.plSpatangoidathe Spatangidæ, an order of petalostichous sea-urchins, generally excluding the clypeastroids or flat sea-urchins