ECHINODERM CHARACTERS 29 1 



spicules, but elsewhere of a series of plates which may be firmly united 

 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 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 (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 repre- 

 sented 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 coelom, 

 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 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. 



