296 



ECHINODERMS OF THE BRITISH ISLES 



2. Echinus Linnaeus. 



Test more or less closely set with spines (tubercles), among 

 which the primaries are often very conspicuous. Three pore pairs 

 in each ambulacral plate. Buccal membrane with few, usually thin, 

 plates, mostly imbedded in the skin, which appears rather naked. 

 Globiferous pedicellarise with the edges of the blade thickened 

 and usually united by cross-beams over the inside, forming thus 



Fig. 167. — Valves of tridentate pedicellariae of Echinus 

 Alexandri (1) and Ech. elegans (2) ; valves of globi- 

 ferous pedicellaria? of Ech. elegans from the inside 

 (3) and in side view (4). Tridentate pedicellaria of 

 Ech. affinis (5). 1-4, x 65 ; 5, x 20. (After Th. 

 Mortensen, ' Ingolf Ech., i.) 



a more or less closed tube; few (1-3) lateral teeth on each side 

 (Fig. 167, 3-4). Chiefly large forms, often of beautiful colour. 



Seven species are known from the British and European seas. 

 They are thus distinguished : 



Key to the species of Echinus known from the British 

 {and European) seas. 



1. A primary tubercle only on every second ambulacral plate, or 



even several plates in succession without a primary tubercle ^ 2 

 A primary tubercle on all the ambulacral plates . . 5 



2. All the interambulacral plates with a primary tubercle . 3 



^ In very young specimens of this section there is a primary tubercle 

 on each ambulacral plate. Such yoimg specimens cannot bo identified 

 alone by the key, and often, indeed, they cannot be identified with 

 certainty. 



