ECHINODERMA— CEINOIDS. 



59 



a peculiar hydraulic system of water- vessels. The structure Gallery 

 of the Crinoidea is illustrated by a series of specimens and ^ Y}^^' 

 drawings, with special reference to fossil forms, and study of 32. 

 this may serve as further introduction to the Echinoderma 

 in general. 



The first point to notice is that in crinoids as in all Table-ease 

 echinoderms, with one or two exceptions, the soft tissues of 

 the animal have the power of depositing crystalline carbonate 

 of lime. This may remain in the shape of minute separate 

 spicules ; or the spicules may grow together into a trellis- 

 work, which forms rods and plates. The deposit is usually 

 most abundant in the skin, where it may be built into a 

 continuous skeleton. Often too, spines of the same substance 

 are borne outside the test. This feature, rare in crinoids 

 but characteristic of the sea-urchin, has given to the sub- 

 kingdom its name Echinoderma, which is a Greek adjective 

 meaning " urchin-skinned." 



The chief parts of the Skeleton of a Typical Crinoid are 

 next shown, and are further illustrated by the accompanying 

 figure (Fig. 28). What one may call the body of the animal 

 is confined to the small portion labelled " cup," on the top 

 of which is the mouth. Since the creature does not move 

 about, it needs some means of bringing food to the mouth, 

 and this is provided by the arms. These are grooved on the 

 inner surface, and water containing the aninialculae on which 

 the crinoid feeds is swept down the grooves to the mouth. 

 The stem serves to raise the cup and arms away from the 

 sea-floor and to sweep them through a larger field of food- 

 supply. 



Perhaps the Crinoidea are descended from animals that 

 were neither fixed nor provided with a hard skeleton. In 

 any case the result of fixation has been with these creatures, 

 as with so many others, the development of radiate symmetry, 

 caused originally by the food-grooves stretching out from the 

 mouth in all directions. Apparently for mechanical reasons 

 connected with the existence of a hard skeleton, the chief 

 planes Of this symmetry have come to be five in number. 

 In other words the skeleton, and to some extent the soft 

 parts and internal organs, can be divided into five similar 

 portions grouped about a central axis. This division into 

 fives or pentamerism, as it is termed, runs right through the 

 Crinoidea and Blastoidea and all the free-moving Echinoderms, 

 although modifications of it arise now and then. It should 

 be understood that the forkinsf of the arms is no modification, 



