THE CCELOM 



of hi^c-h importance. The numerous embryological and anatomical 

 researches of the past twenty years seem to me to definitely 

 establish the conclusion that the cffilom 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 

 cwlom to have originated by a closing or shutting off" of that 

 portion of the general archenteron of Enterocoela (Calentera) 

 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 crelom 

 a primitive elementary ccelom, and originated from the cells of 

 the archenteric wall. 



Probably more than one pair of such cojloms were formed in 

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

 ontogeny of animals with paired coelomic pouches) gave rise to 

 larger continuous cavities. 



The coelom is thus essentially and primarily (as first clearly 

 formulated by Hatschek) the perigonadial cavity or gonoca?l, 

 and the lining cells of gonadial chambers are ccelomic epithelium. 

 In some few groups of Ccelomocoela the creloms 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 m 

 number and of individually large size; in others they are 



numerous. , 



In the great majority of Ccelomocoela the coelom 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 ca>lom 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 coelomic fluid. In the 

 ChKtopoda it has attained to similar dimensions and is distended by 

 li(iuid so as to produce tension in the body-wall. In the Arthropoda 

 (which are now generally regarded as traceable to Cha^topod-like 

 ancestors) the ccelom has shrunk back again to relatively small 

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



