Fam. 3. Porcellanasteridae.—Paxillosa which have lost the anus. There is a conical prominence in the centre of the dorsal surface termed the epiproctal cone, and in the interradial angles there are vertical grooves bordered by folds of membrane produced into papillae, the so-called "cribriform organs." The marginal plates are thin and form the vertical border of the thick disc. The tube-feet have no suckers.
Fig. 203.—Porcellanaster caeruleus. A, aboral view; B, oral view, × 1. (From Wyville Thomson.)
Comparing the Porcellanasteridae with the Astropectinidae we see at once that the "epiproctal cone" is a permanent representative of the temporary aboral elevation in Astropecten, and we are inclined to suspect that the cribriform organs are grooves lined with cilia which keep up a respiratory current like the ciliated spines of Luidia. In all probability the Porcellanasteridae are more habitual burrowers than even the Astropectinidae.
Ctenodiscus (Fig. 196), a genus in which there is a short epiproctal cone and numerous feeble cribriform organs in each interradius, is found in deep water north of the Shetland Islands. Porcellanaster (Fig. 203) is a more typical genus, with one large cribriform organ in each interradius. Hyphalaster has long arms, on which the supero-marginal plates meet above.
Order IV. Valvata.
The Starfish included in this order are characterised by the absence of prominent spines and by the superficial covering of minute granules. The skeleton consists, in most cases, of plates, and these plates with their covering of granules probably represent the first stage in the evolution of paxillae.
The tube-feet possess well-developed suckers. No members of this order can properly be said to be British.
Fam. 1. Linckiidae.—Valvata with long arms, the marginals being developed equally throughout the whole length. These Starfish are distinguished by their long narrow arms and small disc. It is possible that these forms, so different in many respects from the other families of the order, have been directly derived from the long-armed Echinasteridae. Ophidiaster, Nardoa, Linckia.
Fam. 2. Pentagonasteridae.—Valvata with short arms, the marginals being especially developed at the base and in the interradial angles. The aboral skeleton consists of close-fitting plates. Pentagonaster (Fig. 204), Astrogonium.