Besinration and Cell Energy. 
415 
Another property of the liquid is that of absorbing carbon-dioxide and 
giving it up again on exposure to the air. It is necessary, there- 
fore, for this hquid to course through the tissues and afterwards to be 
exposed to the air, and it is sufficient if this exposure takes place through 
a very thin skin. In this way oxygen is provided for the living cells and 
carbon dioxide carried away. The liquid thus described is, of course, blood, 
and the cells in it are known as corpuscles, the colour of the haemoglobin 
being red. With increased complexity of structure then, in animals, 
another requirement become apparent, that is, provision for the exposure 
of the blood to an oxygen-containing medium. Another requirement is a 
means for keeping up a continual flow of the blood through the tissues. 
In worms there is a blood system in which the heemoglobin is dissolved 
in the liquid. The circulation is provided for by several pairs of contractile 
sacs found in certain places in the course of the blood system. There are 
no specialised organs by which exposure of the blood takes place. The 
blood is carried to the sub-epidermis of the whole body, throughout which 
it is distributed by a ramifying system of capillaries. The epidermis and 
cuticle are sufficiently thin to allow of the diffusion of gases between the 
air and the blood. 
It would appear that blood almost from its first appearance assumed 
another important function besides that of a respiratory medium ; that is, 
of carrying nutritive material in solution to all parts of the body. This 
food supply, of course, enters the blood from the seat of digestion in an 
assimilated form. In animals with a blood system the decomposition 
products of metabolism are in greater variety than those of plants, and it 
is necessary for these products to be removed from the blood in which 
they have been dissolved. For this purpose another organ has arisen 
called an excretory organ, or kidney. All blood eventually passes through 
the kidney, and in its course has the effete products, chiefly urea, removed 
from it, and afterwards these are removed from the body. In simpler 
animals, such as the worm, the kidney is represented by one or more tubes 
called nephridia. 
In the Oyster family parts of the body are specialised for exposure of 
the blood to the surrounding medium, i.e., water. These specialised 
organs are known as gills and consist of filamentous structures in contact 
with the water, and in which the blood circulates in fine capillaries ; and 
thus a large surface is provided for exposure. A constant flow of water 
is kept up over the gills by the surface of the gills being covered with 
vibratile cilia. Circulation in the blood system is provided for by a single 
muscular contractile sac called a heart, which in this family is of a simple 
type. In Lobsters and allied animals gills are also found in the form of 
feather-like structures enclosed in a sort of gill- chamber by a portion of 
the calcified cuticle. A flow of water is kept up over the gills by the 
