ECHINODERMA TA 



67 



food of the Urchin. This lantern is very complex, consisting 

 of twenty -five separate pieces arranged in five sets of five. 

 Its detailed description is out of place here, but it is an 

 interesting piece of work to try and dissect it out entire, and 

 study its mechanism. 



Class IV. : HOLOTHURIA 



Sea- The Echinoderms also include the curious tough- 



cucumbers, bodied Seorcucumbers or Holothurians, which, at first 

 sight, look like thick worms rather than 

 relatives of the sea-urchins and star- 

 fishes. Nevertheless, on examination, 

 they are found to possess isolated cal- 

 careous deposits in the leathery skin, 

 and to have five rows of tube-feet down 

 the elongated body, though these tube- 

 feet are only very slightly retractile ; 

 round the mouth, at one end of the 

 body, is a circle of contractile tentacles ; 

 there is therefore an approach towards 

 radial symmetry externally, but this is 

 obscure internally. 



The Sea -cucumber perhaps most 

 commonly found on our coasts is the 

 little white or brown Cucumaria ladea 

 (Fig. 35). These curious little creatures 

 live at the sea-bottom, usually beyond 

 low tide mark, crawling slowly over the 

 ground, and feeding on small molluscs and crustaceans. 

 Another larger British deep-water form is the Cotton Spinner 

 (Holothuria nigra) ; this may grow to the length of a foot, 

 and has a dark upper side and a yellow lower side on which it 

 crawls, only the tube-feet of the under side forming suckers 

 to aid in locomotion. These sea-cucumbers are well known 

 in China, where they are called " trepangs," and form a much 

 relished article of food. 



Class v.: CRINOIDEA 



Sea-lilies or '^^^ °°^y other group of Echinoderms is that 

 Feather of the Feather Stars or Sea-lilies, the Crinoidea. 

 Stars. This is a group which used to be much larger and 



Pig. 35. — Sea-cucumber 

 (Gumi/maria lactea). 



te, Tentacles ; tf, tube-feet. 



