STRUCTURE OF RESPIRATORY ORGANS. 267 
surface may be increased to almost any extent compatible with 
the free access of air to the cavities, and of blood to the walls. - 
In the same manner, where the respiratory membrane is pro- 
longed outwardly, so as to form gills, which hang from the 
exterior of the body (as is the case in most aquatic animals), 
its surface is very much extended by disposing it in folds, and 
by dividing these folds into fringes of separate filaments. It 
has been calculated that, by this kind of arrangement, the 
gills of the Skate present a surface four times as great as that 
of the Human body. 
313. The structure and arrangement of the Respiratory 
organs differ, according as they are destined to come in con- 
tact with air in the state of gas, or to act upon water in which 
a certain amount of air is dissolved. In the former case, we 
usually find the respiratory membrane (which is but a pro- 
longation of the skin or general envelope) forming the wall of 
an internal cavity,—just in the same manner as the membrane, 
through which the act of absorption takes place in animals, 
is prolonged from the skin so as to form the wall of the 
digestive cavity (§ 14). Such a cavity for the reception of 
air into the interior of the body, exists in all air-breathing 
animals ; and in the Vertebrata it receives the name of lung. 
On the other hand, in animals that breathe by means of water, 
the respiratory surface is prolonged externally, so as to be 
evidently but an extension of the general surface,—just in the 
same manner as the roots of plants are prolonged into the soil 
around them. These prolongations, termed branchie or gills, 
which may have various forms, carry the blood to meet the 
surrounding water, and to be acted-on by the air it contains ; 
and then return it to the body in a purified condition. 
314. The form and arrangement of the gills vary greatly 
in the different classes of aquatic animals. Sometimes they 
simply consist of little leaf-like appendages, which have a 
texture rather more delicate than that of the rest of the skin, 
and which receive a larger quantity of blood. In other in- 
stances, they are composed of a number of branching tufts, 
which are more copiously supplied with vessels. Among 
_ the ANnNeELIDA we observe a great variety in the mode in 
_ which these tufts are disposed; and this is connected with 
_ the general habits of the animal. Thus in the Serpula (fig. 
_ 145), whose body is inclosed in a tube, the tufts are disposed 
