616 ECH1NODEEMATA 



Projecting from the surfaces of the radii into the surrounding 

 water are soft, tubular structures known as the ambulacral appendages 

 or podia, which are parts of the ambulacral system (Fig. 966, 10). 

 In many echinoderms a sucker disc terminates each appendage; the 

 animal attaches this to objects in the water and then by the muscular 

 contraction of the appendage pulls itself slowly along; such ap- 

 pendages are called ambulacral or tube feet. Ambulacral append- 

 ages perform a variety of functions, however, besides that of locomotion ; 

 they may assist in the taking of prey and are also always, whether pro- 

 vided with sucker discs or not, important tactile, respiratory, and perhaps 

 excretory organs. Spines also, sometimes controlled by muscles and 

 sometimes not, as well as other special structures, project from the body 

 of starfishes, sea-urchins, and brittle-stars. 



Special sense organs are very poorly developed. Some starfishes, 

 holothurians, and sea-urchins have pigment eyes, and most sea-urchins 

 and some holothurians have static organs, but these and all other echino- 

 derms must depend principally on their tactile sense for a knowledge of 

 their environment. 



The color of the body is hi many echinoderms brilliant and varied, 

 often being orange, red, or purple; some are quite transparent. 



Internal Structures. The body wall of echinoderms contains charac- 

 teristic calcareous bodies (Fig. 966,14). These are least developed among 

 the holothurians, where they are minute spicules, which give a certain 

 rigidity to the body wall, and are best developed in the sea-urchins, in 

 which they consist of polygonal plates, forming a closed case. The body 

 cavity is voluminous and contains the vegetative and reproductive organs. 

 The alimentary tract is well developed and extends from the mouth to 

 the anus (6). The ambulacral system is an extensive system of tubes, 

 which are filled with a watery fluid containing blood or lymph corpuscles. 

 The main part of the system is a ring-shaped canal or tube (4) which 

 surrounds the oesophagus, and five radial canals (7) which spring from 

 the ring canal and traverse the five radii. From these radial canals, and 

 in some cases from the ring canal also, branch canals go off to the ambu- 

 lacral appendages (10), which, as we have seen, project from the surface 

 of the body into the water. Sac-like expansions of the branch canals 

 called ampullae (8) are usually present. From the ring canal an addi- 

 tional vessel called the stone canal (5), because its walls usually contain 

 minute calcareous bodies, passes, in starfishes, sea-urchins and brittle- 

 stars, to the external surface of the body, where it opens to the outside 

 through a perforated plate called the madreporite (9), and in holothurians 

 and crinoids ends free in the body cavity. In crinoids and some holo- 

 thurians more than one stone canal may be present. The purpose of the 



