Prof. G. J. Allman on the Anatomy of Acton. 149 
to trace the anatomy of the apparatus under consideration. We 
can however have no difficulty in recognising in the system now 
described a heart and vessels. That the vascular ramification ob- 
served upon the upper surface of the foliaceous expansions is a 
system of branchial vessels, there can indeed be no doubt, and the 
expansions themselves must therefore be considered as true aéra- 
ting organs. It will at once be seen too, that the ventricle must 
be sought for in the reticulated organ just described, though, from 
the difficulty of the investigation, I have been unsuccessful m my 
attempts to discover its direct connexion with the vessels. ‘The 
circular vessel also must be considered as performing the func- 
tions of an auricle, and it is indeed by no means improbable that 
what appears under the microscope as simply a circular canal 
surrounding the ventricle, is really the projecting margin of a de- 
licate transparent auricle, whose central portion is rendered invi- 
sible by the stronger and more opake ventricle. 
The general opacity of the tissues im -dct@on, the extreme te- 
nuity of the walls of the vessels, and the transparency of the fluid 
which these contain, render the investigation of the circulatory 
apparatus a subject of great difficulty. If too much reliance be 
placed on the compressor, it will certainly escape detection ; and I 
have no doubt that the abuse of this instrument will account for 
the fact of M. de Quatrefages having denied the existence in Ac- 
teon of a vascular system, as well as for many other errors into 
which this naturalist has fallen. 
Nervous. system : Sensation.—Soon after the cesophagus leaves 
the buccal mass it passes through a system of seven ganglia, 
Pl. VI. n, and Pl. VII. fig. 1, of which six are arranged im three 
symmetrical pairs, and one is azygous. 
Of these ganglia, the two largest, Pl. VII. fig. 1 aa, are placed 
upon the upper surface of the cesophagus, being in contact with 
each other iternally. The ganglia of the second pair, b 4, are 
placed immediately below the first, upon the sides of the ceso- 
phagus, being thus separated to allow of the passage of this tube. 
The third pair, ec, seems like a protuberance upon the under side 
of each of the ganglia last mentioned, and the azygous ganglion, 
d, occupies an inferior and median position, completing the sub- 
esophageal portion of this ganglionic collar. The different parts 
of this system of cesophageal gangha are maintained in union by 
three commissures. One commissure, ee, sprigs from each of 
the large ganglia on the upper surface of the cesophagus, and 
then running down along the side of this tube, terminates in the 
azygous ganglion; while the third commissure, f, runs trans- 
versely beneath the cesophagus, uniting the two ganglia of the 
second pair. The cesophagus thus passes between the great 
supra-cesophageal ganglia above, and the transverse commissure 
below. Two small spherical bodies, Pl. VI. 0, most probably 
