ASTEROIDEA 101 



devours also molluscs and echinoderms. No animal is known 

 to feed upon it. Development unknown. 



In British seas it occurs all round the southern, western, and 

 northern coasts. In the North Sea it is known as far south as Hartle- 

 pool. It is distributed elsewhere from the Shetland Islands to 

 the Mediterranean. Bathymetrical distribution ca. 10-200 m. 

 Exceptionalh' (eastern Mediterranean) it goes down to 600 m. 



A hybrid between this species and Asterina gihbosa has been 

 found in the Mediterranean {Pahnipes Lobiancoi Ludwig). 



III. Family Pterasterid^ 



Dorsal side with paxillae, covered over by a membrane (the 

 supradorsal membrane), supported by the spines of the paxillae 

 and thus forming a roof over the whole dorsal side. In this mem- 

 brane numerous small pores (spiracles) occur, which may be opened 

 and closed spontaneously. In the middle of the dorsal side there 

 is a larger opening, surrounded by five valves, supported by some 

 larger spines. Along the edge of the disk usually a distinct 

 fin -like membrane (the " actino-lateral " membrane), formed 

 by a series of long, slender spines, connected by a membrane. 

 Outside the adambulacral plates a series of segmental apertures 

 opening into the supradorsal cavity, and guarded by a specialised 

 opercular spinelet or papilla. No oral interradial plates. 



The supradorsal cavity acts as a brooding pouch, the eggs 

 developing here. There is thus no free, pelagic larval stage. In 

 some species {e.g. Pteraster ohscurus) the young sea- stars do 

 not leave the cavity until they have reached a very considerable 

 size — ca. 1-5 cm. diameter. The cavity likewise has a respiratory 

 function ; the water is sucked in through the segmental pores on 

 the oral side and flows out mainly through the large central 

 opening on the dorsal side, but also through the small spiracles 

 in the supradorsal membrane (which were formerly erroneously 

 stated to be the inhale nt pores). 



Two genera known to be represented in British seas, but two 

 more may be expected to occur there also, viz. Diplopteraster 

 Verrill, with the species D. {Retaster) multipes (M. Sars), known 

 from Skagerrack to the Barents Sea, and from Greenland to 35° N. 

 on the American coast ; also in the Northern Pacific from the 

 Bering Sea to Japan and California, and even recorded from 

 S. Africa (ca. 100-1170 m.) ; and Cryptaster Perrier, with the 

 species Cr. personatus Perrier (Azores, 2994 m.). These two are 

 therefore included in the key to the genera, while not otherwise 

 mentioned in the text. 



