110 STARFISH GALLERY. 



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 pedicellarm ; they 

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

 and the valves sessile. Considerable difficulty attaches to the deter- 

 mination 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, while M. Prouho has observed their use as organs of defence. 



Echinoderms move but little ; the unstalked Crinoids, if they 

 cannot find stones or worm-tubes around which to attach themselves, 

 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 podia or so-called ambulacral feet, 

 which become erected by being filled with water, and are then 

 contracted ; by means of this contraction movement is efi'ected ; 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 20). 



Echinoderms are often of exceedingly bright colours, as is shown 

 by the pictures on the wall, 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 in- 

 jured parts is one of the most remarkable points in the Echinoderm 

 organization (see Case 6). 



Echinoderms are of great geological age, and were very abundant 

 in earlier periods of the world's history. Two groups (the Blastoids 

 and Cystids) have completely disappeared, and the Stalked Crinoids 

 (Lily-Encrinites) are far less common than they used to be. The 

 visitor should make a point of seeing the specimens exhibited in 

 Gallery VIII of the G-eological Department. Echinoderms are now 

 found in all seas, and extend to great depths of ocean ; many of 

 the species have exceedingly wide areas of distribution, and most are 

 characterized by their gregarious habits, a large number of specimens 

 of a single species being generally obtained by the dredge. They are 

 most abundant in the tropical seas. 



Most Echinoderms lay their eggs in the water, where the larvaj 



