1891.] The Evolution of the Circulatory Organs. 239 
reduced to two. D is a diagram of the heart of the Octopus, 
where for the first time we meet with the organ bent upon itself 
E represents the heart of ventricle and single auricle of the Gas- 
tropod, the extreme of development in one direction. 
The heart of the fishes likewise consists of two cavities, one 
auricle and one ventricle, but this is not to be homologised 
with the two cavities of the molluscan heart. In the Mollusca 
the auricle receives aérated blood from the respiratory organs, 
and passes it to the ventricle, which propels the oxygenated 
blood throughout the body, thus forming a systemic circula- 
tion. The Mollusca have no capillaries save in the respiratory 
organs, so that the blood, after leaving the arteries, flows 
through canals or lacune within the substance of the body. 
In the fishes, on the contrary, the two cavities convey only 
venous blood, thus performing the same function as the right 
side of the heart of mammals. The deoxygenated blood is 
gathered up from all parts of the body, and conveyed by the veins 
to the auricle, thence to the ventricle, which organ forces it through 
the truncus arteriosus into the capillaries of the gills, where 
he blood is oxygenated by the free oxygen held by the water. 
The now aérated blood is gathered up by the radices aorte, 
- and the dorsal aorta disttributes it throughout the body (Fig. 4.) 
This figure should be compared with Fig. 2, when it will at once 
be seen, after making allowances for the inverted position of the 
worm, that the heart of the fish corresponds with the dorsal ves- 
sel of that figure, the gill circulation to the transverse vessels of 
the worm, and the ventral vessel of the one to the dorsal aorte of 
the other. The resemblances to the Mollusc are largely those 
of analogy ; those to the Annelid, those of true homology. 
In the reptiles we see a further development of this central engine 
of life. Here we have two auricles with one ventricle. The auricle 
of the fish has had a septum placed down its middle, forming two 
cavities. In some lower forms this septum is incomplete, but in 
typical reptiles it is complete. The ventricle also has the rudi- 
ments of a septum. Indeed, in some of the higher reptiles, the 
crocodile for instance, the separation is almost perfect, thus 
approaching the normal condition of the bird and mammalian 
Am, Nat.—March.—4. 
