BURNE: RENAL ORGANS OF NUCULA NUCLEUS. 303 
important of these are: one, extending upwards and inwards beside 
the rectum and between it and a coil of the intestine (Fig. 3, ard. 1), 
and two that run downwards and forwards, embracing the hindermost 
gut coil (Fig. 4, avd. 2). Shortly after giving off these diverticula the 
pericardiac arm of the organ bends back upon itself to form the 
excurrent arm (Figs. 3 and 4, exc. a.). The latter is quite short, 
somewhat inflated, and without arborescent processes. Near its hinder 
end it opens to the exterior below the attachment of the gill by a short 
uro-genital cloaca that passes above the cerebro-visceral connective. 
From the convexity of the U several arborescent processes are given 
off, and ramify amongst the viscera, extending some way alongside 
the stomach (Fig. 1, arb. 8). The reno-pericardiac duct is of some 
length. It opens into the pericardium by a nephrostome (Figs. 1 
and 2, neph.) situated in the floor on a level with the hinder margin 
of the ventricle, and slightly in advance of the external renal orifice. 
From the nephrostome it passes backwards, internal to the ureter, to 
open into a little diverticulum in the floor of the pericardiac arm of 
the kidney, on a level with the anterior limit of the communication 
between the two organs. 
The renal epithelium, which lines all parts of the organ except the 
reno-pericardiac duct and ureter, consists of large rounded cells, con- 
taining vacuoles and a few concretions. The cells are provided with 
a few long straggling cilia. The reno-pericardiac duct consists of 
a low columnar epithelium covered with remarkably long coarse cilia. 
The epithelium of the ureter is strongly ciliated, and a similar ciliated 
epithelium extends for some distance upon the body-surface, around 
the uro-genital opening. 
Without doubt these kidneys are of a very primitive type, as shown 
particularly by their continuous excretory lining. There has been 
some little discussion as to whether the U shape with anterior con- 
vexity is also a primitive feature. It seems that as regards Nucula 
this condition may very well be a modification, indirectly produced by 
mechanical means, of what was admittedly in all probability the 
original form of lamellibranch kidney, a simple tube leading directly 
from the pericardium to the exterior. In Mucula, and, I believe, in 
other protobranchs, the stomach is remarkable for its relatively great 
size, and extends backwards between and below the anterior horns 
of the pericardium. The pericardium, on the other hand, is peculiarly 
short and broad, forming a crescentic sac with forwardly directed horns. 
The heart likewise is antero-posteriorly compressed, with long spindle- 
shaped auricles that run forwards to the extremities of the pericardial 
horns. This peculiar form of the pericardium and heart can be most 
rationally explained as the result of pressure exerted by the enlarged 
stomach, whereby the central part of the pericardium has been pushed 
backwards and compressed, while the lateral parts are held fast in 
front by the attachment of the auricles to the efferent vessel in the 
gill axis. The nephrostomes, which lie not far on either side of the 
mesial line of the pericardium, would share in its backward migration, 
and as the reno-pericardiac ducts pass outwards in front of the 
ureters, the latter also would be involved in the backward movement, 
