6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



there is the anterior coelome, which is sometimes single {Asterina) 

 and sometimes double {Echinus), the hydrocoele, which is probably 

 fundamentally double, though in some cases • (holothurians and cri- 

 noids) only one hydrocoele sac is formed, and the posterior coelome, 

 which is alv/ays paired. But in those chordates in which the entero- 

 coelic origin of the coelome is clearly presented these three divisions 

 of it always come off from the enteron separately, while in the 

 echinoderms the enteron at most gives off only one pair of coelomic 

 sacs; and whereas in the chordates the middle (collar) coelome is 

 never more closely associated with the anterior than with the posterior, 

 in the echinoderms it is always closely associated with the anterior 

 coelome, being developed from it and remaining connected with it by 

 the stone canal throughout life. 



Whatever its origin the coelome is clearly homologous in all these 

 types so that the manner of its development may be considered as due 

 to special mechanical or other limitations imposed by conditions in 

 the early stages — size. form, amount of yolk. etc. — and not to phy- 

 logenetic causes. 



THE WATER VASCULAR SYSTEM 



The most extraordinary structure of the echinoderm body is the 

 water vascular system. This arises as a narrow dorsolateral out- 

 growth from a portion of the coelome which unites with an ecto- 

 dermic infolding on the anterior aboral surface. From this develop 

 the stone canal and the madreporites. The ectodermic opening places 

 the hydrocoele in communication with the exterior, so that the organ 

 has often been compared, in whole or in part, to an annelid excretory 

 organ or nephridium. 



But Professor Patten has pointed out that it is much more like 

 one of the typical excretory organs of the arthropods (shell gland, 

 green gland, coxal gland) which consist of thin walled coelomic sacs 

 with a thick walled tubular outgrowth of varying length united to 

 a short duct infolded from the ectoderm. 



The five primary tentacles or tube feet of the echinoderm larva 

 according to Professor Patten represent five modified thoracic ap- 

 pendages ; an outgrowth of the underlying somite grows into each 

 appendage in typical arthropod fashion, but instead of breaking up 

 into separate muscles for the appendage it remains permanently in 

 the form of a membranous diverticulum of the hydrocoele and becomes 

 the distal end of a radiating water vascular canal. Only the distal 

 end of the original appendage separates from the body as the primary 



