RESPIKATION. 



565 



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

 less numerous than the tracheas 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- 

 tor}- 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 branchiae 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, w r hich 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 arborescent gills 

 are situated on certain 

 segments only. 



FIG. 240. TRANSVERSE SECTION OF ARENICOLA, AFTER GEGENBAUR. 

 (Jeffrey Bell.) 



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

 dorsal vessel; v', visceral vessel ; P, 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 b}' cilia. 



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

 respiratory appendages are nothing more than thin plates, within which 



