ECHINODERMATA. 57 



spout-like mouth to take up the sand and debris on which they 

 move, and from which they extract some nutriment. Ophiuroids 

 live on the smaller Foraminifera ; Asteroids on dead fishes (as line- 

 fishermen well know), Oysters, and other Mollusks, and even on 

 specimens of their own particular species ; Holothurians on shell 

 or coral debris and the minuteorganismsit contains; and Crinoids 

 on small tests of Foraminifera and on the adults of small and larva? 

 of larger Crustacea. 



In a number of Echinoids and Asteroids some of the spines are 

 specially modified to act as seizing-organs — the free end being 

 divided into two, three, or rarely four pieces, which are moved on one 

 another by special muscles. These minute organs were regarded 

 by earlier observers as parasites, and were named pedicellarice ; they 

 may be movable, when they have a stalk, or the stalk may be 

 absent and the valves sessile. Considerable difficulty attaches to the 

 determination of the use that these organs may be to their 

 possessors; but there is reason to suppose that they may act 

 as cleansing-organs, by removing minute particles of dirt, and 

 as temporary organs of fixation. 



Echinoderms move but little; the unstalked Crinoids, if they 

 cannot find stones or worm-tubes ai'ound which to attach them- 

 selves, swim by beating the water with their delicate arms, five 

 being raised and five depressed alternately. The Echinoid or 

 Asteroid is able to move by the aid of its suckers or so-called 

 ambulacral feet, which become erected by being filled with water, 

 and are then contracted ; by means of this contraction movement is 

 effected ; a similar kind of locomotion obtains with the pedate 

 Holothurians ; in the Ophiuroids the flexible arms either serve as 

 the organs of movement, or act as an apparatus whereby the creature 

 becomes coiled round the branches of corals (see Case 4d). 



Echinoderms are often of exceedingly bright colours, and are 

 very conspicuous objects; this may, apparently, be associated with 

 disagreeable tastes or odours ; sometimes they cover themselves 

 over with seaweed, and so hide their brilliancy; the spines of some 

 forms are exceedingly painful to the touch, and the stout plates of 

 some of the Goniasters must form admirable organs of protection. 

 The power of restoring lost or injured parts is one of the most 

 remarkable points in the Echinoderm organization. 



