CHAPTER XXIV 



THE FOOD OF ANIMALS— OMNIVOROUS ECHINODERMS, 

 SPONGES, AND ANIMALCULES — ANIMALS WHICH 

 FEED LIKE GREEN PLANTS 



ECHINODERMS (Echinodermata) 



The large and characteristic phylum of Echinoderms (p. 153) 

 embraces Star-Fishes, Sea-Urchins, Sea-Cucumbers i^Holothti-- 

 rzans), and Crinoids (Sea-Lilies and Feather-Stars). The first 

 of these are essentially carnivorous, and have already been dealt 

 with (p. 153), but the remainder, being largely omnivorous, 

 claim some attention here. They exemplify feeding by sand- 

 swallowing and the action of ciliary currents, both being common 

 modes of obtaining nutriment, as has already been shown. 



Sea-Urchins (see vol. i, p. 456) are spheroidal or flattened 

 forms, covered with movable spines, and with the mottth placed 

 on the under surface. Five pointed jaws, which grow continu- 

 ously, like the front teeth of a rabbit, are often but not always 

 present. The thin coiled intestine is, in many species, found 

 on dissection to be full of sand, from which the organic matter is 

 extracted just as in a Lob- Worm (p. 257) or Siphon- Worm 



(p- 259)- 



Sea-CucuiMBERS iyHolothurians) (see vol. i, p. 462) are worm- 

 like animals with thick leathery skins. The motctk is placed 

 at the front end, and is surrounded by a circlet of tentacles, the 

 shape of which varies a great deal in different species. They 

 are generally used for shovelling sand or mud into the mouth, 

 but in some cases are long and branched, so that when fully 

 extended they can be employed as a sort of net in which food 

 floating in the surrounding water gets entangled. 



The commonest existing Crinoid is the Feather-Star (Co7n- 

 atzila) (see vol. i, p. 460), which consists of a central disc, from 

 which ten feathery arms radiate, the plume-like appearance of 



