318 



LECTURE XV. 



in the Gasteropodous Mollusca : the ventricle is exclusively systemic, 

 and is provided with only two venous apertures. Four such apertures 

 are described and figured by Dr. Lund on the dorsal surface of the 

 heart, but these are regarded by Audouin and Edwards as depres- 

 sions merely between the muscular fasciculi, having no communica- 

 tion with the ventricular cavity. 



I have tested the conflicting evidence of these observers by dissec- 

 tion of the heart in the lobster ; and you will perceive by this prepa- 

 ration* that it is more complicated than even the Danish Naturalist 

 supposed, and fully bears out the opinion of Hunter in regard to the 

 mixed nature of the circulation in the Crustacea. 



Fig. 133. shows the heart laid open on the ventral aspect; by ex- 

 posing its cavity from this side, it was 

 readily determined whether the exterior 

 depression on the dorsal surface did or did 

 not lead into the cavity. On either side of 

 the anterior depression from which the 

 ophthalmic (/) and the antennal {g) arte- 

 ries arise, there is a large oblique fissure, 

 guarded by a pair of semilunar valves. The 

 vein, which terminates by each of these 

 orifices, is an extended, irregular, and ex- 

 tremely flattened sinus, lying above the 

 heart, and between it and the lining mem- 

 brane of the shell, communicating anteriorly 

 with a similar broad irregular flattened sinus, which occupies a simi- 

 lar situation above the stomach, and receiving posteriorly the blood 

 from a series of flat and expanded sinuses which correspond to each 

 segment of the tail. There are two similar openings at the sides of the 

 ventricle, posterior to the preceding, which conduct the blood from la- 

 teral sinuses into the heart : these orifices, indicated by the bristles 

 (c, c), have each a pair of semilunar valves. The arterial blood, re- 

 turned from the branchiie, enters the heart by the large orifices on the 

 sides of the ventral aspect (<?, c?), regurgitation being likewise prevented 

 by two semilunar valves at each orifice. The muscular fasciculi of the 

 ventricle are so arranged as to leave tolerably distinct chambers for 

 each series of arterial orifices. The strongest bands or columns are 

 transverse, and dorsad of the branchial apertures : anterior to these is 

 a chamber receiving the carbonized blood from the two anterior dorsal 

 apertures; it is partially divided, the upper chamber giving off (/)the 

 ophthalmic, and {g) the antennal arteries. The lower chamber sends 



Astacus marinus. 



* No. 898. A. 



