246 The American Naturalist. [March, 
which in most reptiles remains open. A still greater degradation 
in its character has been occasionally evinced, for several cases 
are now on record in which the heart has preserved but two 
cavities, an auricle and a ventricle, thus corresponding with that of 
the fish; and in one of these instances the child has lived for 
seven days, and its functions had been apparently but little 
disturbed. 
The bifed character of the apex, which presents itself at an 
early period of the development of the mammalian heart, and is 
permanent in the Dugong, sometimes occurs as a malformation 
in the adult subject, evidently resulting, like the others which 
have been mentioned, from an arrest of development. 
The Blood-—The form-elements of the blood itself indicate a 
parallel evolution with that of the heart and vessels: In the 
Vermes, where the vascular system is first separated from the 
digestive tract, the liquid contents known as the blood are generally 
colorless, occasionally green or reddish in color, and the form- 
elements are of but slightly different cells. The blood of the. 
Echinoderma (sea-stars and sea-urchins) is of clear or slightly 
opalescent color, and the form-elements are simple cells. 
The blood of the Arthropoda is generally colorless; only in a 
few insects is it greenish or reddish; even then the color is due 
to the plasma and not to the cells, which are colorless and of 
variable size and form, and absent entirely in some of the lower 
forms, ás. the Crustacea. The blood of the Mollusca is generally 
colorless, sometimes bluish, violet, or green ; only in one species 
is the blood red, and then from the plasma, for the blood-cells are 
simple, undifferentiated, and always colorless. 
The blood of crabs and other Crustacea has been proved by M. 
Fredericq to contain the same saline elements and the same strong 
and bitter taste as the waters they inhabit. But the blood of sea- 
fishes is very different. It has not the same constitution as that 
of the crabs, and shows a marked superiority over them. In 
fact, the character of the blood-fluid of the invertebrates is 
strikingly similar to the lymph of the higher vertebrates where the 
lymphatic and vascular systems are separated. In both, the 
cells are simple and undifferentiated, colorless, opalescent, or pink. 
