NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK 7 



tentacle ; the remainder of the appendage, however long it may event- 

 ually become, may be regarded as lying in the surface ectoderm, de- 

 veloping on either side as it increases in length paired cirri which 

 become the double row of tube feet for each arm into each of which 

 a prolongation of the water vascular canal extends. 



Primarily, then, each of the five primitive thoracic appendages, one 

 from each of the five half metameres of which the echinoderm body 

 is composed, contains a tube of nephridial intent leading into a canal 

 opening to the exterior by a pore. The anterior and posterior ends 

 of the series of half metameres join, and the excretory canal becomes 

 a ring canal from which grow out five long radial canals giving ofif 

 branches to the tentacles or tube feet as these are formed. 



The excretory function of the water vascular system of the echino- 

 derms is reduced to a minimum if, indeed, it can be said to exist at all. 

 Its action is chiefly that of an hydraulic system whereby power origin- 

 ating in a great number of weak and dissociated muscle fibers scat- 

 tered along and within the water tubes and their branches is unified 

 and transmitted to the hollow tube feet, tentacles, and other appen- 

 dages, while at the same time the constant inflow and outflow of 

 water through the madreporic openings, especially when these com- 

 municate with the body cavity as they do in the gill-less crinoids, 

 undoubtedly serve to a greater or lesser extent the purpose of 

 respiration. 



THE ECHINODERM VASCULAR SYSTEM 



The vascular system, which is especially well developed in the 

 holothurians and echinoids, is formed of a peculiarly modified con- 

 nective tissue in which the fibers are sparse and which contains inter- 

 communicating spaces without an epithelial lining. The fluid in these 

 spaces does not appear to undergo any definite movement. Typically 

 there is a circumoral tract with radial prolongations which lie be- 

 tween the radial water vessel and the radial nerve cord, an annular 

 aboral tract in which the generative rachis is embedded and which 

 sends oflf extensions to the genital organs, and in holothurians and 

 echinoids a considerable development in the mesentery and on the 

 gut wall. 



In the barnacles no heart is ever present, and the lacunar channels 

 in which the blood circulates are for the most part ill defined. 



