Prof. G. J. Allman on the Anatomy 0/ Actseon. 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 j there can indeed be no doubt, and the 

 expansions themselves must therefore be considered as true aera- 

 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 in 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 in Actceon, 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- 

 tion of a vascular system, as well as for many other errors into 

 which this naturalist has fallen. 



Nervous system : Sensation. — Soon after the oesophagus leaves 

 the buccal mass it passes through a system of seven ganglia, 

 PL VI. n, and PI. VII. fig. 1, of which six are arranged in 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 oesophagus, being in contact with 

 each other internally. The ganglia of the second pair, b b, are 

 placed immediately below the first, upon the sides of the oeso- 

 phagus, being thus separated to allow of the passage of this tube. 

 The third pair, c c, 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- 

 oesophageal portion of this ganglionic collar. The difierent parts 

 of this system of oesophageal ganglia are maintained in union by 

 three commissures. One commissure, e e, springs from each of 

 the large ganglia on the upper surface of the oesophagus, and 

 then running down along the side of this tube, terminates in the 

 azygous ganglion ; while the third commissure, /, runs trans- 

 versely beneath the oesophagus, uniting the two gangha of the 

 second pair. The oesophagus thus passes between the great 

 supra-oesophageal ganglia above, and the transverse commissure 

 below. Two small spherical bodies, PL VI. 0, most probably 



