THE ANATOMY. SPERMIDUCAL GLANDS 117 



the exterior ; in spite of this difference, who could deny that the structui-e which I 

 have called ' atrium ' in Branchiura is the exact homologue of the ' atrium ' in other 

 Tubificidae? To suppose that two structures so similar and yet morphologically 

 different could exist in the genera of a limited family like that of the Tubificidae 

 is to suppose too much, and to go counter to plain facts. Still, if Benham was 

 right in calling the gland appended to the male duct in Pontodrilus ' prostate,' and 

 the similar gland in EvArilus ' atrium,' it will be necessary to explain how it is 

 that structures which are of different morphological import have come to possess an 

 almost identical structure. And if the term 'prostate' is retained 'for those glands 

 which either pour their secretion- into the sperm-duct or open independently to the 

 exterior,' then the structure which I have called 'atrium' in Branchiura cannot be 

 the homologue of the ' atrium ' in other Tubificidae and Lumbriculidae. This appears 

 to me the reductio ad ab&urdum, and to dispose of the necessity for further argument. 

 There has been, however, some little confusion as to the meaning to be attached to 

 the term 'prostate,' independently of the facts ali-eady referred to. I have called the 

 glandular investment of the ' atrium ' in the Lumbriculidae ' prostate,' and compared 

 this glandular investment to the ' Cementdrusen ' of Tuhifex : it may be that I have, 

 as Benham suggested, compared together structures which cannot be compared, since 

 they are respectively epiblastic and mesoblastic in origin; in any case I did not 

 seriously make that comparison after deliberation; I am now inclined to think that 

 that comparison is after all the right one, provided only that embryology confirms 

 it. The glandular investment of the ' atrium ' in the Lumbriculidae does not appear 

 to be peritoneal ; we cannot, however, be certain, though it is so exactly like the 

 glandular investment of the ' atrium ' in Moniligaster which is probably not peritoneal. 

 I have seen sections through the ' atrium ' of Moniligaster in which the thick layer 

 of cells investing it externally is covered by a thin layer of what is undoubted 

 peritoneum ; and there is no doubt that the layer of cells in question opens by 

 prolongations of the cells into the lumen of the gland — a state of affairs which is 

 not suggestive of its being in reality peritoneum, as I at first believed it to be. 

 Vejdovsky, too, has figured a similar prolongation of the cells of the glandular 

 coating of the ' atrium ' in Ehynchelmis to join the lumen ^ ; the same thing undoubtedly 

 exists in Sutroa ; in all these cases, therefore, I believe that we have to do with 

 a second layer of epithelium of epiblastic origin; the absence, if it be ultimately 

 proved, of a peritoneal layer in certain Lumbriculidae, and in some species of 

 Moniligaster does not appear to be a matter of the greatest consequence, since in 



' Moreover, Benham has described a delicate membrane surrounding the pear-shaped cells which is 

 probably the coelomic epithelium. 



