74 



ZOOLOGY. 



are composed of a number of branched filaments, and resemble 

 little trees or vascular branches (a a, Fig. 56). 

 Finally, in others, they are formed in small 

 membranous lamellae, disposed like the leaves 

 of a book, or like the teeth of a comb. The 

 first of these arrangements takes place in 

 many marine animals, as in the arenicola, so 

 common on the coasts. The second may be 

 observed, also, in several of the annelida and 

 in some of the Crustacea. Finally, the last 

 is common to most molluscan animals and 

 fishes. 



It is also to be observed, that in the in- 

 ferior animals the branchiae are mostly situated 

 externally, so as to float freely in the sur- 

 rounding water; whilst in the more highly 

 organized, as the mollusks and fishes, the 

 branchiae or gills are enclosed in a cavity, into 

 which the water has free access, and may 

 easily be renewed. 



133. Organs of Aerial Respiration : 

 Respiration in Air. The organs serving for 

 this form of respiration affect sometimes the 

 form of trachea, sometimes of lungs. 



The tracheae (Fig. 56) are vessels which 

 communicate with the exterior by apertures 

 called stigmata, and ramify in the depth of 

 the organs. They convey the air to these 

 organs, and thus the function of respiration 

 is carried on in every part of the body. This 

 mode of breathing is peculiar to insects and 

 to some arachnidae (spiders). 



134. The lungs are pouches, more or less 

 divided into cells or cellules, which also re- 

 ceive air into their interior, and whose walls 

 are traversed by vessels containing blood, thus 

 exposed to the vivifying action of the air. 



There exist lungs, but in a state of the 

 greatest simplicity, in most spiders; and in 

 . some mollusks, as in snails. Keptiles, birds, 

 Sea Worm and mammals also have lungs. 



L Worm ot ?S 135 - In man ' as in al1 mammals > the 

 Fishermen, lungs are lodged in a cavity, called the thorax, 



F - Th 



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