ECHINODERMA— CRINOIDS. 
59 
a peculiar liydraulic system ol' water-vessels. The structure 
of the Crinoidea is illustrated by a series of specimens and 
drawings, with special reference to fossil forms, and study of 
this may serve as further introduction to the Echinoderma 
in general. 
The first point to notice is that in crinoids as in all 
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 Slceleton of a Typical Crinoid are 
next shown, and are further illustrated by the accompanying 
figure (Fig. 28). AVhat 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 animalculae 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-tloor and to sweep them through a larger field of food- 
supi.ly. 
Perha])S 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 t»f radiate symmetry, 
caused originally by tlie food-grooves stretching out from the 
mouth in all directions. A|)parently 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 arul 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 
Ije understood that the forking of the arms is no moditication. 
Gallery 
VIII. 
Table-case 
32 . 
Table-case 
32 . 
