2) 
218 MR. R. OWEN ON THE STRUCTURE OF THE HEART 
Mollusks. But at the point, or trunk, where the two venous trees are united we find 
no heart interposed, the respiratory circulation in these Invertebrata being in this respect 
analogous to the portal circulation in the Vertebrate classes. Even in the Dibranchiate 
Cephalopods, where the respiratory apparatus is perfected by the development of a mus- 
cular ventricle appropriated to the lesser circulation, it is interesting to observe that 
this organ is not placed at the point of divergence of the branchial vessels from the 
great central vein, but is, as it were, divided, and a branchial heart is placed at the 
base of each gill. I have on a previous occasion! pointed out the dependence of this 
superadded complexity upon the superior locomotive energies, and the related per- 
fections of the nervous system which the Dibranchiate Cephalopods enjoy. 
The still higher developed muscular powers of Fishes necessarily demand that the 
circulation through the respiratory organs in them should, in like manner, be aided by 
the propelling power of a ventricle. If the branchial and pericardiac cavities of a Hep- 
tatrema, Dum., be laid open and compared with the corresponding parts of a Sepia, it 
would seem as if the two branchial hearts of the Cephalopod had been approximated 
and united at the median plane in the Fish, while the arteries remained separate, each 
diverging from the other, and supplying the gills of its respective side. In Petromyzon 
the lower or posterior half of the branchial artery continues single, or conjoined. In 
other Fishes the mesial conjugation extends throughout the branchial trunk. The 
heart, however, which in the Mollusks is appropriated to the immediate reception and 
distribution of the aerated blood, has disappeared in Fishes. The gills in this class 
being so subdivided as to be subjected to effectual and constantly repeated pressure 
of the surrounding parts, the blood is driven more forcibly out of them than in the 
Cephalopods, where they float loosely in a large cavity. Again, the proportion which 
the muscular parts of the Fish bear to the visceral cavity is much greater than in the 
Mollusk, and therefore the systemic circulation derives more effectual assistance from 
the general contractions of the body; and it is this circumstance principally, though 
doubtless aided by the structure and allocation of the gills, which renders a ventricle 
for the greater circulation unnecessary in Fishes. 
It is well known that the more complex heart of the higher Vertebrata is developed 
from, or at an early stage has a structure analogous to, the simple heart in Fishes, 
and that, at first, its force is in like manner immediately exerted to propel the blood 
through branchial vessels, but is afterwards gradually concentrated upon the aorta by a 
series of obliterations of these vessels. In Siren, Proteus, Menobranchus, and Awolotes 
the stream issuing from the ventricle is still considerably subdivided in the external 
branchie ; and in consequence of this resistance to its passage additional means are 
provided to prevent regurgitation into the ventricle. In Menopoma the stream is di- 
verted into eight undivided channels before passing into the aorta; in Amphiuma it is 
carried from the heart to the descending aorta along four equally simple channels ; in 
' Memoir on the Nautilus, p. 50. 
