THE CCELOM 



of high importance. The numerous embryological and anatomical 

 researches of the past twenty years seem to me to definitely 

 establish the conclusion that the coelom is primarily the cavity, 

 from the walls of which the gonad cells (ova or spermata) develop, 

 or which forms around those cells. We may suppose the first 

 co3lom to have originated by a closing or shutting off of that 

 portion of the general archenteron of Enteroccela (Ccelentera) 

 in which the gonads developed as in Aurelia or as in Cteno- 

 phora. Or we may suppose that groups of gonad mother- 

 cells, having proliferated from the endoderm, took up a position 

 between it and the ectoderm, and there acquired a vesicular 

 arrangement, the cells surrounding a cavity in which liquid 

 accumulated. 



It is not of importance for our present purpose to decide be- 

 tween these two possible origins. They only differ in the earlier, 

 or later development of the cavity which the gonad mother-cells 

 surround. 



In whichever of these two ways the cavity took its origin as a 

 separate chamber distinct from the archenteron, it was a coelom, 

 a primitive elementary coelom, and originated from the cells of 

 the archenteric wall. 



Probably more than one pair of such cceloms were formed in 

 the primitive Coelomoccela, and by their fusion (as occurs in the 

 ontogeny of animals with paired coolomic pouches) gave rise to 

 larger continuous cavities. 



The ccelom is thus essentially and primarily (as first clearly 

 formulated by Hatschek) the perigonadial cavity or gonoccel, 

 and the lining cells of gonadial chambers are ccelomic epithelium. 

 In some few groups of Ccelomoccela the coeloms have remained 

 small and limited to the character of simple gonoccels. This 

 seems to be the case in the Nemertina, the Planarians, and 

 other Platyhelmia. In some Planarians they are limited in 

 number and of individually large size ; in others they are 

 numerous. 



In the great majority of Ccelomocoela the ccelom has vastly 

 extended its area and acquired secondary functions and a leading 

 importance in the physiology and architecture of the animal. In 

 the adult Echinoderma and Vertebrata, the ccelom is (omitting 

 secondary divisions) a single cavity of very large size, extending 

 in every direction between the body-wall and the gut-wall, and 

 occupied by a specialised fluid the ccelomic fluid. In the 

 Chaetopoda it has attained to similar dimensions and is distended by 

 liquid so as to produce tension in the body-wall. In the Arthropoda 

 (which are now generally regarded as traceable to Chsetopod-like 

 ancestors) the coelom has shrunk back again to relatively small 

 dimensions. It exists in them as the cavity of the gonadial sacs 



