214 SUPPLEMENT 
thick, which have taken up the blood of the posterior parts of 
the body, and even of the edges of the mantle. 
This medial sinus, which is surrounded by a brown organ, 
of which we shall speak further on, when we come to treat of ~ 
urinary depuration, appears also to receive a considerable 
number of vessels from it, or perhaps those vessels may 
originate from the former and proceed to distribute themselves 
to this organ, while a much greater number unite in the 
branchial arteries. These are two in number, one on each 
side. ‘They are considerable, and placed longitudinally along 
the upper edge of the branchial plates, thicker in the middle, 
they diminish in diameter, and finish in a point at the ex- 
tremity in proportion as they have furnished branches to the 
gills. ‘Those branches form two planes, one for the internal 
face of the external leaflet, and the other for the external face 
of the internal leaflet of the gills, descend vertically, dimi- 
nishing as far as the edge of the leaflet, and furnish numerous 
longitudinal anastomosing branches, so that from them results 
a net-work with square meshes. From this same net-work 
spring, by a contrary disposition, the branchial veins, whose 
net-work occupies on each leaflet the face opposite to the 
arterial net-work, and they unite in as many thick longitudinal 
veins as there are branchial plates, at least in front, where 
they are perfectly separated at the superior edge; for behind | 
there are but three, the medial being common to the two in- 
ternal plates, which are united. The external branchial vein 
changes into a sort of sinus or long auricle, with which the 
external vein communicates by several venous pedicles ; and 
this auricle itself, after having been narrowed, opens into the 
ventricle. 
The palliobranch species appear to have the auricles still 
more distinct than the ordinary acephala, and it is probable 
that it is this which has caused it to be admitted that they 
had two hearts. 
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