302 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 
at this point opens into the uro-genital cloaca, just dorsal to the 
opening of the genital duct (Figs. 2 and 8, 2). ‘The opening is 
a simple perforation of the apposed walls of the duct and cloaca, and 
isin no way drawn out to form such a tubular connection as that 
described by Stempell in this position in Leda. There is no doubt 
whatever that the opening of the genital duct is into the uro-genital 
cloaca in front of and below this ‘ gono-pericardiae’ pore, and not, as 
Pelseneer finds in his specimens, into the reno-pericardiac duct. The 
meaning of this connection is somewhat obscure. It is certainly not 
an artifact, for it is present and perfectly symmetrically on both sides, 
besides having been noted in several other genera. Nor is it easy to 
see by what physiological need such an opening would be called 
into being. It would no doubt serve to rapidly remove products of 
excretion from the pericardial cavity, were such present; but in these 
lowly forms the pericardial epithelium is not yet specialized to 
form an adventitious excretory organ (Grobben’s glands, Keber’s 
organ). It is most probably, as Stempel] suggests, the original passage 
of the genital duct into the kidney, in course of being supplanted by 
the direct path through the uro-genital cloaca. 
To turn now to the anatomy of the organs of Bojanus (Figs. 1 and 4), 
which, although not so important as the connection between the kidney 
and the genital duct, is yet of some considerable interest. In the 
descriptions of these organs in the Protobranchia given by Pelseneer, 
Stempell, and Drew, there is a general similarity. Typically, in the 
Nuculide and Ledide each organ is tubular and U-shaped, with the 
convexity of the U directed forward (it is turned backward, according 
to Pelseneer, in Nucula nucleus), and with the pericardiac and external 
openings lying nearly in the same transverse plane at the free end of 
either arm of the U. The tube of which the kidney consists is 
generally very long (Leda, Yoldia), and is more or less sacculated or 
arborescent and coiled. The arborescence is specially marked in 
Malletia. The distal part of the excurrent arm is smooth without 
sacculations. The entire organ, except the reno-pericardiae duct and 
the ureter, is lined by excretory epithelium. In most cases (Leda 
sulcata, L. pella, Malletia Chilensis, Yoldia hyperborea) there is an open 
communication between the pericardiac arms of either side. 
From a comparison of three series of transverse sections of Vucula 
nucleus, it appears that in all essentials the organs of Bojanus in this 
species resemble those of other Nuculide and Ledide. They are in 
the first place lined throughout (except for the reno-pericardiac duct 
and ureter) by excretory epithelium. Each is U-shaped, with the 
convexity of the U directed forward. But the limbs of the U are 
very short, far shorter than as figured by Stempell in Leda sulcata or 
described by Drew in Yoldia, and are not coiled, but give off many 
sacculations and arborescent processes. 
The hinder end of the pericardiac arm of the kidney lies below the 
posterior parts of the pericardium at some little distance in front of 
the posterior adductor. It opens freely into its fellow of the opposite 
side. In front it stretches forward and diagonally outwards below 
the pericardium, and gives off several arborescent processes. The most 
