On the Structure and Functions of the Organs of Respiration. 223 



minute apertures iu the sides of the body ; and it is not a little remark- 

 able that the middle tunic of the three which compose these tubes, should be 

 formed of spiral fibres, which may be drawn out like the spiral vessels of 

 a plant j the tubes being thus provided with elasticity sufficient to prevent 

 them from being obstructed by compression. It is impossible to study 

 the manner in which the air is thus brought into contact with the blood in 

 the minutest textures of the body, without being struck with the method by 

 which the limited circulation of the fluids is compensated by the extensive 

 distribution and complex ramification of the respiratory apparatus, and by 

 which the necessary lightness, elasticity, buoyancy, and muscular energy are 

 imparted to the bodies of these beautiful and interesting inhabitants of the 

 air. It is also worthy of remark, that this very high degree of respiratory 

 power is given, not by a sudden advance to a more complicated system of 

 organs, such as exist in the vertebrated classes of animals ; but by an 

 extension of the comparatively simple plan of which we observed the first 

 traces in the annelides j thus afibrding a beautiful example of the great 

 law of unity of type in the development of organs, which has few apparent 

 and perhaps no real exceptions. 



In many of the arachnides, we observe a state of the respiratory appara- 

 tus which may be considered as a more concentrated form of that of insects, 

 and perhaps as a transition to a higher type. In the spider, for instance, 

 there are numerous distinct sacs opening from the sides of the abdomen, 

 into which the air freely enters j but their interior, instead of being 

 smooth, has a laminated or pectinated character, partaking of the structure 

 of gills, or perhaps presenting an approach to that separation into distinct 

 cells which characterises the lungs of higher animals. From the analogies 

 which they present to both classes of organs, Dr. Audouin denominates 

 them pulmonary branchiaj. 



In the Crustacea, the respiration is aquatic, being performed by pecti- 

 nated gills, which are usually placed on the under surface of the body, and 

 generally in connection with some very moveable parts. In some kinds of 

 Crustacea, as the land crab, there is a special apparatus for keeping the 

 gills in a moist state when at a distance from the water, and thus enabling 

 them to be acted on by air. 



The molluscous classes present great variety in the form and situation 

 of their organs of respiration, although they are all, with but a few excep- 

 tions, inhabitants of the water. Most of them are remarkable for 

 the slowness of their movements, and many arc entirely fixed; and it is 

 beautiful to observe how all of them, even the most inert, are provided 

 with the means of renewing the fluid iu immediate contact with their bodies, 

 so as to aerate and renovate the blood. Although the form and position 

 of the gills varies much in the different classes, their general structure is 

 the same in all ; they consist of delicate membranous folds or tufts, mi- 

 nutely reticulated with blood-vessels, and covered with vibratile cilia, by 



