278 ECHINODERMS OF THE BRITISH ISLES 



2. TiibcieU's c'lonulato ; ocular ])lato.s inoro or loss broadly in contact 



with the |)criproct ....... 3 



Tubercles smooth ; ocular plates excluded from the periproct 



PedinidcB 



3. Genital and ocular ])lates of almost equal size and shape, forming 



a very conspicuous ring round the peri|)roct. Only a single 

 large tubercle on each interambulacral plate. Test high ; 

 spines very long and slender, usually curved 



A sp idodiadematidcB 



Genital and ocular plates not of equal size and shape ; several 



large tubercles at least on the interambulacral plates at the 



ambitus of - test. Shape of test usually somewhat flattened ; 



spines usually very long, but not curved . Diadematidce 



I. Family EcHiNOTHURiDiE 



Test flattened, usually soft and flexible, on account of the fact 

 that the coronal plates are imbricated ; the imbrication is 

 different in the two areas, the plates of the interambulacral areas 

 overlapping from the mouth towards the apex, the ambulacral 

 plates in the opposite direction. The peristome w holly covered by 

 the ambulacral plates, which continue in an uninterrupted series 

 to the very edge of the mouth. The gills are very small ; no gill- 

 cuts in the peristomial edge of the test. Primary spines on the 

 oral side of test club-shaped or tipped with a white hoof. 

 Miliary spines usually provided with poison glands. Polian 

 vesicles usually w^ell developed. 



The Echinothurids are chiefly deep-sea forms, often gor- 

 geously coloured ; in the Indo-Pacific also some littoral forms 

 are found. Some of them reach a very large size, up to more 

 than 300 mm. diameter of test, the largest of all known 

 Echinoids. When being dried without special preservation these 

 forms will generally collapse completely and become quite flat. 

 Also they are liable to be more or less distorted by the rough 

 handling in the trawd, and especially the large primary spines 

 with the characteristic terminal hoof are very often broken, so 

 that it may be difficult enough to find even a very few of them 

 intact. The poison spines sting very painfully. Development 

 unknown, but judging from the large size of the eggs the deep- 

 sea forms at least must have a direct development, without a 

 pelagic larval stage. 



Four genera are known from the British seas (and the N.E. 

 Atlantic). 



