XVI HABITS AND DISTRIBUTION 429 



Brittle Stars and Sea-cucumbers, which can be dredged up from 

 below low-water mark, and lastly the beautiful Feather-stars, of 

 which there are comparatively few species still living, although 

 huge beds of limestone are composed of the remains of fossil 

 Feather-stars. 



One species of Sea-cucumber (Synapta similis) ^ is said to enter 

 brackish water in the mangrove swamps of the tropics ; but, 

 with this exception, the whole phylum is marine. A few species 

 can endure partial exposure to the air when left bare by the 

 receding tide, but the overwhelming majority are only found 

 beneath low-water mark, and a considerable number live in the 

 deepest recesses of the ocean. 



Their distribution is, no doubt, partly determined by food, 

 a number of species being strictly confined to the neighbourhood 

 of the shore. On the other hand, since a very large number 

 of species live on the layer of mud impregnated with animal 

 remains which forms the superficial layer of the deposit covering 

 the sea-floor, it is not surprising to learn that many have an 

 exceedingly wide range, since this deposit is very widely dis- 

 tributed. Another equally important factor in determining 

 distribution is wave-disturbance, and it is surprising to learn 

 to what a depth this extends. Off the west coast of Ireland a 

 large wave literally breaks on a submerged rock 15 fathoms 

 beneath the surface. Speaking generally, it is useless to look 

 for Echinoderms on an exposed coast, and the same species, 

 which in the sheltered waters of the Clyde are exposed at low 

 water, must be dredged up from 20 to 30 fathoms outside 

 Plymouth Sound. 



The ordinary collector is attracted to the group chiefly by 

 the regularity and beauty of the patterns produced by the radial 

 symmetry, but to the scientific zoologist they are interesting from 

 many other points of view. Differing widely nevertheless from 

 the higher Invertebrata in their symmetry when adult, they have 

 as larvae a marked bilateral symmetry, and the secondary 

 development of the radial symmetry constitutes one of the most 

 remarkable life-histories known in the animal kingdom. 



Then again, owing to the possession of ossicles, the Echino- 

 dermata are one of the few groups of Invertebrata of which 

 abundant remains occur fossilised. In attempting, therefore, to 

 1 Ludwig, loc. cit. p. 357. 



