Fig. 248.—Young Echinoneus to show five equal radii scarcely petaloid. amb, Ambulacral area; g.p, genital pore; mad, madreporite. (After Agassiz.)
The Classification of the Spatangoidea is based mainly on the degree of development of the petals, that is to say, on the extent to which the burrowing habit has been developed. But weight is also laid on the shape of the peristome, the pentagonal form being more primitive. Seven families are recognised, which are as follows:—
Fam. 1. Echinonidae.—"Petals" hardly marked at all; peristome in the centre of the lower surface and pentagonal. Floscelle not developed.
One genus, Echinoneus (Fig. 248).
Fam. 2. Nucleolidae.—"Petals" distinct; peristome as in the foregoing family. No floscelle. Nucleolites, with the anus in a furrow. Anochanus, with a concave apical system serving as brood-pouch.
Fam. 3. Cassidulidae.—"Petals" usually distinct; peristome eccentric, but provided with a well-marked floscelle.
Echinolampas, with the anus on the under surface.
Neolampas, with the anus on a projecting papilla. One specimen of this genus has been dredged in the British area.
The three foregoing families probably use their tube-feet to walk with, and bury themselves only to a slight extent. They are often united as a sub-order, the Asternata, and distinguished from all the rest which possess an eccentric mouth and well-marked plastron. These families are then grouped together as Sternata. They are as follows:—
Fam. 4. Ananchytidae.[[503]]—Spatangoidea with elongated apical system, ambulacra all similar and not grooved. Petals feebly marked. Pourtalesia, with bottle-shaped posterior prolongation of the test. Platybrissus, with flattened test.