866 SPECIAL PHYSIOLOGY. 



Certain fishes, as the Loach, can swallow air, which probably enters the air- 

 bladder ; but, in other cases, and especially when this organ forms a closed sac, 

 air must be alternately excreted from the blood and absorbed by it, according 

 as the bladder is full or empty. In the Amphioxus, the alimentary canal aids 

 in the respiratory process. The analysis of the air in the air-bladder some- 

 times shows a large proportion of carbonic acid, and sometimes little more 

 than nitrogen. In a few species, in which it is of great size, and communicates 

 freely with the pharynx, the air-bladder may be regarded as a feeble sub-respi- 

 ratory organ. In the great majority of cases, however, it is a rudimentary 

 part, exercising little or no respiratory function ; it is large in the Flying-fishes, 

 and in some others capable of energetic and sustained exertion. As elsewhere 

 mentioned (p. 187), when distended it diminishes the specific gravity of .a Fish 

 supported in water, and also alters the centre of gravity of the animal. It is 

 usually absent in what are called ground-fishes, which live in deep water ; but 

 its presence or absence appears to follow no precise rule, either as regards the 

 habits, size, or generic position of the Fishes in which it exists or is wanting. 



Mollusca. The Pulmogasteropods offer examples of Non-vertebrated aerial 

 breathers ; they are terrestrial in their habits. In the snails, for example, a 

 large sac communicating with the external air by an aperture situated on the 

 left side of the neck, is found in the shelled varieties beneath the back part of 

 the mantle, in that portion of the body which occupies the smaller coils of the 

 shell. It has numerous bloodvessels ramifying upon its walls, and is usually 

 lined with cilia. This simple sac may be taken to represent the primitive idea 

 of a lung, that is to say, a sac formed by an inversion of the surface of the 

 body, lined by a thin moist membrane communnicating with the air, and pos- 

 sessing, distributed upon its walls, bloodvessels, which have thin coats and a 

 constantly moving blood-current in them. Such an organ becomes perfected 

 by increase of size, by multiplication of its parts, so as to constitute a multi- 

 lobular lung, by subdivision of its internal surface into saccules or air-cells, 

 and by the penetration of air-tubes into its numerous lobules. The remaining 

 Gasteropods, and, indeed, all other Mollusca and Molluscoida, breathe aquat- 

 ically. 



Annulosa. Numerous instances of air-breathing animals occur in this Sub- 

 kingdom. Entire Classes, without exceptions, such as the Myriapoda, the 

 Arachnida, and the extensive and most important Class of Insects, breathe in 

 this way. Of the Annelida a few only are terrestrial and respire air ; these, 

 such as the earth-worms, have pairs of small sacculi opening on the sides of 

 the body in each segment ; but they are usually filled with mucus. Similar 

 structures exist in the leech. The air-breathing Annulosa, generally, how- 

 ever, do not breathe by soft membranous compound sacs like lungs, or by soft 

 air-bladders, or even by simple soft sacculi like those of the land-snails ; but 

 they have internal air-chambers, surrounded by stiff walls, so that they are 

 kept constantly patent, and are not easily compressed. 



In the Myriapoda these chambers are saccular, a pair of symmetrical sacs 

 existing in each of the many segments of the body ; they communicate by short 

 membranous tubes, the walls of which exhibit spiral lines with little apertures 

 on the sides of the segments, named spiracles or stigmata. From these sacs, in 

 one species, forming a step towards the condition of Insects, other short twigs 

 ramify into the body, or large symmetrical lateral trunks connect them all to- 

 gether. In this way air is conveyed into close proximity with the nutrient 

 fluids, and respiratory interchanges are accomplished. 



In the Arachnida air-sacs exist in the spiders, communicating immediately 

 with spiracles or stigmata on the surface of the body, and frequently having 

 their internal membrane plicated, so as to increase the surface for exposure to 

 air, for the aeration of the fluids of the body. This plicated structure is asso- 

 ciated with a more perfect condition of the circulation, and compensates for 

 the comparatively limited distribution of the air in these active and powerful 

 animals. In other Arachnida, the air-chambers are tubular, constituting 

 trachea, as found in the mites. 



The Insects possess in greatest perfection this particular modification of an 

 aerial respiratory apparatus, viz., a series of almost incompressible canals or 

 tubes, instead of sacs. In these exceedingly active and energetic animals, 



