THE ANATOMY. COELOM 25 



III. COELOM. 



The coelom in the Oligochaeta is invariably spacious and nearly invariably divided 

 into compartments which correspond with the external metamerism. The division is 

 effected by means of the intersegmental septa which are only wanting in Aeolosoma ; 

 these septa are not as a rule applied to the parietes in such a way as to exactly 

 correspond with the grooves on the exterior of the body that mark the segments ; 

 hence the internal metamerism is not precisely as the external metamerism. The 

 coelom is lined throughout by the peritoneal epithelium, which is reflected over all 

 the organs that lie within it ; from its wall are developed the gonads ; it communicates 

 with the exterior directly by means of the dorsal pores, and indirectly by means of 

 the nephridia and the genital ducts. 



The coelom of the Oligochaeta is developed out of the paired mesoblastic masses 

 which are formed early in the embryo ; each pair joins its fellow in the mid-dorsal 

 and mid-ventral line ; but there is a nearly complete fusion above and below. No 

 longitudinal septum remains to mark the division of each compartment of the coelom 

 into right and left halves, and on the ventral side there is the mesentery supporting 

 the ventral vessel only, which represents the ventral part of the line dividing the 

 right and left halves. It has been stated that in Criodrilus the dorsal septum is 

 persistent, but this appears to be an error. As a general rule, the coelom shows 

 no signs of division into different cavities, except of course by the intersegmental 

 septa; but this rule has a few exceptions. In the first place, the sperm- sacs and 

 egg-sacs are undoubtedly portions of the coelom enclosed by special walls, which 

 are set apart for the maturation of the sperm and ova respectively ; these structures 

 are dealt with under the description of the reproductive organs. In addition to these 

 there are the remarkable sacs which envelop the gonads and the spermathecae in 

 many Eudrilids, and which form so marked a character of that family. These 

 sacs for the most part play the part of spermathecae, otherwise for the most part 

 wanting in the family Eudrilidae, and they are dealt with under the description of the 

 spermathecae. 



The lining membrane of the coelom varies greatly in its characters in different 

 regions of the body ; but it is nowhere ciliated, as is the case with other worms 

 (e. g. the Archiannelida). The parietal layer is composed of flattened cells, the nuclei 

 of which can as a rule be alone recognized in transverse sections ; this epithelium 

 can be demonstrated by the silver method with great success (see POWER). 

 CLAPAREDE too has figured the flattened coelomic epithelium upon the^ nerve cord, 

 though, as already mentioned, VKJDOVSKY has stated the non-continuity of the coelomic 



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