ASTEROIDEA. 225 



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 

 foimation 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 Antedon this 

 apical system is fully represented, except that the infra-basals are 

 reduced to three, but in other Crinoids and in the adult Antedon there 

 tends to be reduction. Among other Echinoderms the apical system is 

 best represented among sea-urchins, where there are often five basals 

 and five radials arranged around the anus, but these tend to be reduced 

 or lost among the modern irregular urchins. In Ophiuroids the apical 

 system is sometimes represented both by basal and radial plates, but 

 often only by radials ; in star-fishes 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 offbranches 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 ampullae. 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. Star-fishes. 



The description applies especially to the common five- 

 rayed star-fish (Asterias or Asteracanthion rubens). It is 

 often seen in shore pools exposed at low water, but its 

 haunts are on the floor of the sea at greater depths. There 

 it moves about sluggishly by means of its tube feet. 



Each of the five arms bears a deep ventral groove in 

 which the tube feet are lodged. The mouth is in the 

 middle of the ventral surface, the food canal ends about 



15 



