254 PHYLUM ECHINODERMA. 
together, as in most sea-urchins, or may be capable of movement upon 
one another. Apart from the skeleton proper, lime may appear in 
almost any of the organs of the body. With this deep-seated tendency 
to form skeletal substance may perhaps be associated the sluggish habit 
of the majority, and the absence of definite excretory organs, Except 
in Holothurians, where the calcareous plates are diffusely scattered, the 
parts of the skeleton show much regularity of arrangement. The 
primitive skeleton is believed to have consisted of two series of plates, 
constituting respectively the oral and apical systems. These, especially 
the latter, were of much importance in the formation of the skeleton of 
the extinct Blastoids and Cystoids, but in modern Echinoderms they 
are absent or unimportant, and are functionally replaced by accessory 
plates, such as those which form the ‘‘test” of sea-urchins, The oral 
system consists of five plates surrounding the mouth, and in living forms 
it is fully developed only among Crinoids. The apical system in the 
Pelmatozoa typically forms a cup or calyx enclosing the viscera, and 
consists of a central plate to which a stalk may be attached, and three 
sets of plates arranged around this, five infra-basals, five basals, and five 
radials. In the larva of Aztedon this apical system is fully represented, 
except that the infra-basals are reduced to three, but in other Crinoids 
and in the adult Azzedon there tends to be reduction. Among other 
Echinoderms the apical system is best represented among sea-urchins, 
where there are often five basals (the genitals) around the anus. The 
‘‘oculars” seem to correspond to the ‘‘terminals” at the tips of 
starfish arms. In Ophiuroids the apical system is sometimes re- 
presented both by basal and radial plates, but often only by radials ; 
in starfishes it is typically absent in the adult, though more or less 
clearly shown in the larva. 
The other most striking characteristic of Echinoderms is the peculiar 
water vascular system. This arises in development from the ccelom, 
and consists typically of the following parts:—An external opening or 
madreporite opens into a canal with calcified walls, called the stone 
canal; this opens into a ring canal around the mouth, which has often 
connected with it little vesicles and glandular bodies; the ring canal 
opens into five radial canals which run in the radii of the body, and 
give off branches to the protrusible tube-feet which project on the 
surface of the body, and may be furnished with suckers; the radial 
canals are also often connected with internal reservoirs or ampull. 
The tube-feet are very characteristic, and have different functions in 
the different classes. In Asteroids, in most Holothurians, and in part 
in Echinoids, they are primarily locomotor ; in Ophiuroids, in Crinoids, 
and in part in Echinoids, they are respiratory, tactile, or used for food- 
catching. But there is great variety of structure and functions; thus in 
many Holothurians the tube-feet are represented only by a ring of 
tentacles around the mouth, 
Class ASTEROIDEA. Starfishes 
Star-like or pentagonal Echinoderms more or less flattened 
at right angles. to the main axis of the body ; usually with 
