KESPIRATION. 



565 



In the spider the respiratory apparatus consists of a series of sacs v 

 less numerous than the tracheae of the silk-worm, and not communicat 

 ing with each other ; yet additional space is obtained by arranging the 

 lining membrane into a series of folds, which lie in close relation to each 

 other like the leaves of a book, thus forming the first indication of a lung. 

 From the extensive surface thus produced, in which lies a net-work of 

 vessels, the blood is brought into immediate relation 

 with the air, which enters through the breathing parts 

 referred to. The exchange of air in the sacs is accom- 

 plished by the movements of the body of the insect, 

 which empty the sacs by compression and allow them 

 to refill by the elasticity of their walls. These respira- 

 tory cavities are called pulmonary branchiae from their 

 resemblance on the one hand to the lungs of the higher 

 animals, and on the other hand to the branchise or gill- 

 sacs (Fig. 241). 



In the oyster and mollusk we have an approach to 

 the respiratory apparatus of the fish. On opening an 

 oyster a delicate membrane, known as the mantle, is 

 seen lining the edge of the cell. The gill is constituted 

 by a double fold of the mantle covered with cilia; upon 

 the gill ramify the blood-vessels, which are bathed by 

 the water which passes over them. From this water 

 the blood receives oxygen and gives carbon oxide to it, 



Fig. 239. — The 

 Lob-worm (Are- 

 nicola). (Carpen- 

 ter.) 



The arhorescent gills 

 are situated on certain 

 segments only. 



Fig. 240.— Transverse Section of Arenicola, after Gegenbatjr. 

 (Jeffrey Bell.) 



D, dorsal, V, ventral sides : N, ganglionic chain ; I, intestine ; be. gills ; v, ventral vessels ; D, 

 dorsal vessel ; v', visceral vessel ; r, vessel around intestine ; a b. vessels of gills. 



—an exchange which is just as essential to the oyster as for breathing 

 mammals (Fig. 242). 



In the clam the gills are inclosed in the mantle, forming a tube, the 

 siphon, through which the water is forced by cilia. 



In the lowest forms of crustaceans, as in the branchiopods, the 

 respiratory appendages are nothing more than thin plates, within which 



