;M4 ECHIN0DERM8 OF THE BRITISH ISLES 



little known, and cannot, for the present, he distinguishecl in the 

 II. stage from the corresponding larva? of Echinus esculentus 

 and acutus. 



It is mainly a littoral form, which may bore holes in the rocks. 

 The food consists of all sorts of minor bottom organisms ; Hydroids 

 especially apjiear to be a favourite food. Also algse are eaten, 

 and often the intestine is found to be filled with bottom material. 

 A parasitic mollusc, Pelseneeria stylifera Turton (S^'n. Stylifer 

 Turtoni Broderip), is not uncommon on this sea-urchin, and in 

 its intestine the Planarian Syndesmis echinoncm is found. It is 

 eaten at least by the cod. 



In British seas this species occurs along the North Sea coasts, 

 from the Shetland Islands to the southern part of the North Sea, 

 but has not been recorded from the west coasts. It is elsewhere 

 distributed from the Siberian Sea and Spitzbergen to the Sound, 

 from Iceland and Greenland to New Jersey, and in the Northern 

 Pacific to Vancouver and Korea. Bathy metrical distribution, 

 0-ca. 1200 m. 



Very young specimens of the polyporous Echinoids have as 

 yet only 3 pore pairs in the ambulacral plates, and plates with 

 only 3 pore pairs are still found near the peristome in the adult 

 forms. Such young specimens can, however, be determined with 

 certainty by means of their globiferous pedicellarise, which appear 

 very early. Also the quite young Psammechinus and Echinus are 

 very easily distinguished by means of their globiferous pedi- 

 cellaria?. The structure of the pedicellariae is easih^ seen under 

 the microscope by applying some glycerine or potash, or on 

 dissolving the tissue by means of a drop of hypochlorite of 

 sodium. On living samples of pedicellarise the tissue is usually 

 so transparent that the structure of the valves can be seen 

 directly, without any special treatment. 



III. Order. Clypeastroidea 



Test elongate or round, usually flattened. ]\Iouth in the 

 middle on the under side, anal opening usually on the under side 

 or at the posterior edge, only exceptionally a small distance above 

 the edge. No gills or gill- cuts at the peristomial edge of the test. 

 Interior of test usually with a complicated system of supporting 

 pillars or walls, connecting the ujiper and lower floor ; this 

 supporting system ma}' be developed to such a degree that the 

 internal organs are wholly enclosed by a calcareous network, or 



