224 On the Structttre and Functions of the Organs of Respiration. 



the action of which constant and regular currents are produced. These 

 cilia are also placed around the branchial apertures, and often fringe the 

 margins of the mantle. In some cases, however, the gills are placed ex- 

 ternally to the mantle, occasionally forming membranous processes like 

 fins, as in some of the pteropodes. Many of the gasteropodes are terres- 

 trial, and are consequently modified for aeriform respiration. In the snail, 

 for instance, we find an opening on the right side of the body, which leads 

 to a capacious highly vascular sac destined to receive atmospheric air ; 

 this sac is placed nearly in the middle of the back, the position in which 

 we find the air bag in fishes. 



In this slight sketch of the development of the organs of respiration 

 in the iuvertebrata, we observe that the entire covering of the animal is 

 subservient to this function in the lowest tribes ; but, as we rise higher 

 in the scale, we find a portion of the surface specially modified for the 

 aeration of the blood, forming the gills or pulmonic cavities, which are 

 situated in the most convenient position for receiving that fluid and sub- 

 mitting it to the influence of the surrounding element. It is also interesting 

 to remark, that these organs do not, as yet, communicate with the mouth, 

 which is an organ solely appropriated, in these lower classes, to the recep- 

 tion and subdivision of the food ; and that the movements by which respi- 

 ration is assisted, are, in most cases, those of the body at large. 



There are scarcely any organs which present greater diversities of form 

 among the vertebrated classes, or which undergo more marked changes 

 during development, than those which are appropriated to the aeration of 

 the blood ; and these diversities of form are most intimately connected 

 with the living condition of the animals, and the vital energies they are 

 capable of displaying. In the slow moving reptile, as in the moUusca, where 

 the respiration is small, it may be suspended for a time without inconveni- 

 ence ; but in the active inhabitants of the air, birds as well as insects, 

 where this function is necessarily of great extent, its suspension quickly 

 proves fatal to life. If we keep a bird in a limited quantity of air until 

 it ceases to respire, and then place successively in the same atmosphere a 

 dormouse, a frog, and a snail, each of these animals will continue to breathe 

 for some time in a medium which the former one had vitiated too much to 

 continue to support its own respiration. 



Although the respiratory apparatus in fishes retains the type which 

 characterised the inferior classes, it undergoes great increase, both in 

 extent and importance. In order to keep pace with the rapid advance in 

 the development of the other systems, the respiration requires to be con- 

 ducted, though by means of an aquatic element, with great velocity and 

 eflfect. For this purpose, it is not sufficient that fishes should have merely 

 ciliated tufts, hanging loosely at the sides of the neck ; but it is requisite 

 that they should have the nieans of rapidly and constantly propelling large 

 streams of water over their surface j and of forcing the whole blood of the 



