296 
MR. NEWPORT ON THE NERVOUS AND CIRCULATORY SYSTEMS 
points of interest. Each side of these double lamellae is formed of an exceedingly 
delicate and apparently structureless double membrane, which include within it a 
parenchymatous tissue, formed of single vesicles or cells, in which I have been unable 
to detect any nuclei. These cells exhibit the appearance of simple bodies, from 
which it might well be conceived that vessels might be formed. In some places these 
vesicles are arranged more in distinct series, and are also slightly elongated. The 
whole parenchymatous tissue of the lamina is made up of these cells, which are larger 
and more elongated, and assume a slightly conical appearance near where the air 
enters at the base of each plate, in which part these cells are nearly uniformly distri- 
buted within the double membrane. But in the upper or more convex portion of each 
lamina, numbers of these minute cells are aggregated together in numerous, irregular, 
rounded patches, which thus produce a tuberculated or glandular appearance in the 
lamina. These aggregations of cells are more thickly interspersed through the struc- 
ture of the lamina the nearer they are to its convex margin, where I have sometimes 
seen what I believe to be delicate but exceedingly indistinct vessels penetrating the 
lamina, but which could be followed only for a very short distance into it among the 
cells. The convex margin of each lamina is however bounded by a delicate but 
distinct vessel, which seems to form the means of intercommunication between the 
anastomosing net-work of vessels distributed over the branchiae and the structure of 
the laminae, since the delicate evanescent vessels traced into the laminae are derived 
from those which bound their convex margin. I have also observed vessels extended 
from these marginal vessels on the laminae, which I regard as the anastomoses between 
these and those which cover the whole branchiae, and distribute the blood from the 
portal branches. 
It has already been stated, that at the posterior part of the inner side of the 
branchiae, on their superior internal margin, where the laminae are covered by the 
thick membrane and peritoneum that covers the common cavity of the branchiae, 
there are several small orifices (fig. 31. t), the commencement of vessels which 
afterwards, when collected together, form the larger channels that convey back the 
blood to the heart. At the superior angle of each lamina of the branchial plates 
there is a small sinus that opens into a larger one, formed on this border of the com- 
mon cavity by the whole of the laminae, which from its direction backwards appears 
to communicate with the orifices. From these facts it would appear that the blood 
permeates the nucleated parenchymatous tissue of each double lamella, and is brought 
into contact with the air, when the branchiae are inflated during respiration, by endos- 
mosis through the membranes, and then, collected in the sinuses at their base, it passes 
out again through the orifices and vessels at their superior border, to be returned again 
to the heart. These vessels form delicate vascular trunks or sinuses which pass 
around the sides of the body in the posterior part of each segment, and gradually 
enlarged by communicating with other vessels in their progress, pour their contents 
into the heart at the auricular orifices ( u ) on its dorsal surface. 
