368 OF RESPIRATION. 



they set free is communicated at once to the atmosphere, instead of 

 being taken up by the blood ; and the oxygen they require is imbibed 

 in the same manner. And thus^we see how the respiration of this inte- 

 resting class, which is unequalled for its energy when the bo'dy is in a 

 state of activity, is provided for without an active circulation of blood, 

 and without the presence of red corpuscles, which elsewhere seem to 

 be essential conditions of the interchange of oxygen and carbonic acid 

 between the air and the tissues, wherever this takes place to any great 

 extent. 



661. In the Spider tribe, we return to a more concentrated form of 

 the respiratory apparatus ; but, notwithstanding that it is limited within 

 much narrower dimensions externally, it exposes a very large amount 

 of surface on its interior. It consists of a series of sacs, much less 

 numerous than in the lower Articulata, and not communicating with 

 each other. Their lining membrane, however, is doubled into a series 

 of folds, which lie in proximity with each other, like the leaves of a 

 book, and which thus present a very extensive surface within a very 

 small space. Over this surface the blood is distributed in a minute 

 capillary network ; and thus it comes into immediate relation with the 

 air, which is received into the cavity through its aperture or spiracle. 

 The alternate admission and expulsion of air seem to be provided for, 

 as in Insects, by movements of the body, which first empty the cavities 

 or air-tubes by compression, and then allow them to be refilled by their 

 own elasticity, the pressure being relaxed. The respiratory cavities in the 

 Spider-tribe have received the name of pulmonary Iranchice, from their 

 analogy, on the one hand, with the lungs of higher animals, and, on the 

 other, with the branchial sac or gill-cavity of the higher Crustacea, the 

 gills in which are formed by prolongations of the lining membrane, like 

 the leaf-like folds in the air-cavities of the Spider-tribe. 



662. The accompanying diagram will give an idea of the relations of 

 these different forms of the respiratory apparatus, amongst themselves, 

 and to that of Yertebrata. Let the line AB represent the general 



Fig. 103. 



Diagram illustrating different forms of the Respiratory apparatus : o, simple leaf-like gill ; 6, simple 

 respiratory sac ; c, divided gill ; d, divided sac ; e, pulmonary branchia. 



surface of the animal ; the continuations of that line on its upper side 

 being its external prolongations ; and those below, its internal prolon- 

 gations or reflexions. Now at a is seen the character of the simple 

 foliaceous or leaf-like gill, such as is found in the lower aquatic animals ; 

 presenting merely a flat expanded surface in contact with the water, 

 over which the blood may be distributed. At b is shown a correspond- 



