MR FRANK E. BEDDARD ON" THE ANATOMY OF OCNERODRILUS. 575 



number of segments (as far as the XXXVth) from their external pores. As these organs 

 probably grow inwards from an invagination, various circumstances might easily prevent 

 their growing in one direction or prevent their growth in another, and thus lead to great 

 variations in the amount of the coiling of the tube. 



In one specimen which I examined the atrium of one side of the body was divided 

 into two tubes, which, passing under the nerve cord, lay upon the side of the body 

 opposite to that upon which the external pore was situated. 



The atrium, as shown in Dr Eisen's figures [1, pi. i. figs. 4, 9], is divisible into a 

 muscular and a glandular region ; this is usually the case with the atria in Earthworms, 

 though not in Nemertodrilus [Beddard, 5]. The muscular portion communicates 

 directly with the exterior ; it is lined by a low layer of epithelial cells which is sur- 

 rounded by a thick coat of muscular fibres, chiefly circular in direction. The glandular 

 portion of the atrium is in certain points peculiar. The great length of the atrium, 

 extending as it does as a cylindrical tube through a considerable number of segments, 

 recalls the tubular atrium of Pontodrilus, Acanthodrilus, Dichogaster, and some other 

 genera of Earthworms, and I had expected to find its structure identical with that of the 

 atria of those Oligochseta. I find, however, that in Ocnerodrilus Eiseni, as in Ocnero- 

 drilus occidentalis, the atrium is lined by a single layer of glandidar cells. This differ- 

 ence in minute structure is of some importance. In all Earthworms in which a 

 " tubular " atrium is present its glandular epithelium consists of two distinct strata of 

 cells, which are not unlike those of the clitellum. It is only in the Moniligastridse and in 

 the "Limicolse" that the glandular part of the atrium agrees with the non-glandular 

 portion in having an epithelial lining only one cell thick. Ocnerodrilus and Moniligaster, 

 therefore, render it impossible to utilise this character as distinctive of the " Limicolse," 

 which it otherwise would be. 



But, in any case, the possession of an atrium showing this structure is a point of 

 similarity to the lower aquatic Oligochseta. 



It might be supposed to bear some relation to the small size of the worm were it not 

 for the fact that in Microscolex, which is hardly larger, the atrium has the characteristic 

 structure of Earthworms. 



Fig. 13 of the Plate illustrates a transverse section through the atrium. It 

 is covered externally by a very thin peritoneal layer, which appears to contain a few 

 delicate muscular fibres ; the nuclei belonging to this layer were very evident. The 

 lining epithelium looks at first sight as if it were made up of a layer of large glandular 

 cells only. These cells are somewhat oblong, oval in shape, and have abundant granular 

 contents which are not stained by the colouring reagent used (borax carmine) ; towards 

 the base of the cell is a large spherical deeply-staining nucleus. 



The glandular cells lining the atrium are separated from each other by darkly stained 

 but very thin structures, which are really non-glandular cells. A few of these can be 

 always seen in a given section to be furnished with a nucleus (see fig. 13) placed near to 

 the middle of the cell. 



