Fam. 3. Gymnasteridae.—Valvata allied to the foregoing but distinguished by possessing a very thick skin in which the plates are completely buried. Dermasterias, Asteropsis.
Fam. 4. Antheneidae.—Valvata with short arms. The dorsal skeleton is reticulate and each ventral plate bears one or several large valvular pedicellariae (Fig. 195, C). Hippasterias, Goniaster.
Fam. 5. Pentacerotidae.—Valvata with arms of moderate length. The dorsal skeleton is reticulate but the ventral plates bear only small pedicellariae or none. The upper marginals are smaller than the ventral ones.
The Pentacerotidae include both short-armed and long-armed forms. Amongst the former is Culcita, in which the body is a pentagonal disc, all outer trace of the arms being lost; Pentaceros is a long-armed form.
Fig. 204.—Pentagonaster japonicus. × ⅔. (After Sladen.)
The family Pentagonasteridae furnishes the key to the understanding of most of the forms contained in this order. It contains genera such as Astrogonium which possess on the back unmistakable paxillae, whilst on the under surface they have the characteristic covering of granules; these genera seem to be closely allied to the short-armed species of the Archasteridae, from which they are distinguished chiefly by the granular covering of the marginals. From a study of these cases it seems clear that the plates of the dorsal skeleton of the Valvata correspond to the supporting knobs of the paxillae much broadened out, and the granules correspond to the spinelets of the paxillae increased in number and diminished in size.
As mentioned above, Ludwig has proved that the paxillae develop in the life-history of the individual out of ordinary plates, the axis of the paxilla representing the plate.
Order V. Forcipulata.
This order, which includes the most highly developed members of the class Asteroidea, is at once distinguished by the possession of forcipulate pedicellariae which, as we have seen, possess a well-marked basal piece with which the two plates articulate. The pedicellariae are consequently sharply marked off from the spinelets, and no intermediate forms occur. The first conjoined adambulacrals, which in other orders form the "teeth" or mouth-angles, do not here project beyond the first pairs of ambulacral plates.