DESCRIPTION 617 



stone canal is to introduce sea water (or the body fluid) into the ambu- 

 lacral system. Tlie fluid in it is, however, never pure sea water but 

 always contains lymph bodies in the form of amoeboid cells. Lymph 

 glands occur in the ambulacra! system of many starfishes in the form 

 of small sacs called Tiedemann's vesicles, which spring from the wall of 

 the ring canal. In many echinoderms there also arise from the ring canal 

 elongated sacs known as Polian vesicles. These seem to serve chiefly as 

 regulators of the pressure in the ambulacral system, but they may also 

 give rise to the amoeboid cells. An organ of somewhat problematical 

 function called the axial organ, which surrounds the stone canal, without, 

 however, opening into it, is also probably a lymph gland, and has, besides, 

 a close relation to the reproductive organs; in most echinoderms this 

 organ is itself surrounded by the axial sinus, a tubular portion of the body 

 cavity. 



The fluid in the ambulacral system is kept in circulation by means of 

 cilia on the inner surface of the tubes, and muscle fibers in their walls; 

 valves are present where the ambulacral feet join the branch canals. 



There are two or three systems of nerves in the echinoderm body. 

 These are (1) the superficial oral system, consisting of a ring nerve 

 around the mouth or the oesophagus, and five radial ner\'es which pass 

 along the five radii; (2) the deeper oral system which accompanies the 

 superficial system; and (3) the apical system which is confined to the 

 aboral body wall. The first of these systems is present in all echinoderms 

 and innervates the ambulacral system and the digestive canal. The second 

 is wanting in crinoids and certain sea-urchins; the third is especially 

 prominent in crinoids, but is wanting in holothurians. 



The general surface of the body, including the ambulacral append- 

 ages, plays the greatest part in the performance of the functions of res- 

 piration and excretion. In starfishes and sea-urchins, where the surface 

 is ciliated, its importance in this particular is increased; the papulae of 

 the former and the peristomial gills of the latter still further enlarge its 

 functional area. Most holothurians have special respiratory organs, and 

 the bursae of brittle-stars may function as such. 



The body cavity is not a single space but a complex system of spaces 

 and canals, some of which are often cut off from the rest, and which 

 contain a cireulatoiy fluid similar to that in the ambulacral system. There 

 is one principal space, which may be called the body cavity proper, con- 

 taining the gi-eater part of the viscera, and small spaces and canals which 

 extend to every part of the body and surround the principal organs. The 

 most important of these canals are the radial blood vessels, which consist 

 of a ring vessel and five radial vessels, following the ambulacral canals 

 and the radial nerves. No heart is present, and the blood fluid is kept in 



