240 ANIMALS OF THE SEA FLOOR 



urchins (Fig. 171), and their kindred are abundant in 

 all seas. Belonging to the same group (Echinodermata) 

 are the sea-cucumbers, trepangs, or Holothuroidea 

 (Fig. 172), and the feather-stars (Crinoidea), some of the 

 latter free-living like the Antedon of our own seas, some 

 attached by a stalk like the " stone-lilies " of earlier 

 geological epochs (Fig. 175). These stalked crinoids, 

 confined for the most part to fairly deep water, were 

 formerly among the most prized rarities of zoological 

 collections, though the progress of deep-sea exploration 

 has now rendered them tolerably familiar. 



Under the name of worms are included very many 

 animals of widely dissimilar appearance. The Pla- 

 narians are mostly flattened leaf-like forms, often of 

 brilliant colours, which glide smoothly over the surface 

 of stones or shells. The Nemertines may be recognized 

 by their soft, very extensile bodies, and their long 

 threadlike proboscis, which can be completely with- 

 drawn within the animal. They are frequently richly 

 coloured and of very varied form, some slender and of 

 immense length, others short and thick and very 

 fragile, breaking up into small fragments on the least 

 provocation. Burrowing in mud or clay we find the 

 Echiuroids, or spoon-worms, and the Sipunculoids ; the 

 latter with tough leathery bodies, blunt at the tail, 

 and tapering to a small proboscis, which, as in the 

 Nemertines, can be withdrawn into the body like the 

 finger of a glove which is turned inside out. 



By far the most important and numerous group of 

 worms is that of the bristle-worms, or Polychaetes, 

 which are to be found practically everywhere in salt 

 water (Fig. 178). They carry stumpy, footlike lobes 

 beset with bristles along each side of the body, by 

 means of which they crawl or swim, and breathe by 

 gills which are either situated on the lateral lobes, or 



