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FRANK E. BEDDARD. 
tively the homologues of the glandular region of the atrium in 
Acantliodrilus, and the groups of glandular cells are the 
homologues either of the Cementdriisen of Tubifex or of the 
glandular covering of the atrium in Rhyne helm is and 
Moniligaster. The crucial fact, however, which to my 
mind settles the latter homology, is the presence of 
a delicate peritoneal layer surrounding the whole 
organ. In Moniligaster and in Rhynchelmis, &c., there 
is no peritoneal layer surrounding the prostates, for the very 
sufficient reason that the prostates are themselves the modified 
peritoneal cells. All the structures, therefore, which lie within 
the peritoneal layer must belong to the atrium; the so-called 
prostate of Perichseta is therefore not the homologue 
of the glandular investment of the atrium of Rhyn- 
chelmis and Moniligaster. 
There are some difficulties in the way of a comparison 
between the prostate of Tubifex and that of Perichseta. Itis 
true that there is a very considerable superficial similarity. The 
origin of the prostate in the Tubificidae from the epithelium of 
the atrium has been followed by both Yejdovskv and Eisen; 
these facts therefore are in favour of the comparison. On the 
other hand, it seems on a priori grounds likely that the 
prostate of Perichseta is the homologue of the atrium in 
Acanthodrilus ; in this case, as already pointed out, the 
glandular cells must correspond in both genera ; in the 
Tubificidae the atrium is lined by a single layer of cells, some 
of which become modified into the prostate gland; the super- 
added glandular layer of the atrium is altogether wanting. I 
should be inclined, therefore, for the present to regard the 
prostate of Tubifex as not strictly homologous with 
the so-called prostate of Perichseta. 
Although the structure of the “ prostate gland ” in the 
majority of Perichetse is like that of P. Houlleti described 
above, and presents therefore considerable resemblances to the 
prostate glands of the Tubificidae, this is not always the case. 
In a species (P. Newcombei) which I have recently de- 
scribed from Australia, and which is probably identical with 
