VALVATA 



471 



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 Star- 

 fish 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. Ophidiastw, 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. 



Fam. 3. Gymnasteridae. Valvata allied to the foregoing 

 but distinguished by possessing a very thick skin in which the 

 plates are completely buried. Der master ias, 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 



