264 



ECHINODERMS OF THE BRITISH ISLES 



among tlio most important and common fossils. From the 

 Cretaceous formations of northern Europe they were during 

 the Glacial epoch scattered over the ground through the 

 action of the ice. Various superstitions attach to such earth - 

 found " thunder-stones ". (Of. above, p. 9.) 



Fig. 147. — 1-2. Larvae of Psammechinus miliaris in the I. stage, in side 

 view (1) and seen from the oral side (2). 3. Fully formed larva 

 (II. stage) of Echinus esculentus, from the dorsal side (slightly dia- 

 grammatic), xca. 120. {¥totii Danniark''s Fauna.) 



b, Dorsal arch ; ep, Anterior epaulettes ; epi, Posterior epaulettes. 



Several Echinoids protect their brood ; but all the littoral 

 forms occurring in British and European seas have pelagic larvae. 

 These larvae, Echinopluteus, are provided with a very complicated 

 skeleton. Two larval stages may be distinguished, the I. stage, 

 with only two pairs of arms, the postoral and antero-lateral 

 arms, and the II. stage, with the full number of arms (Fig. 147). 

 In the larvae of regular Echinoids the body skeleton is partly 



