RESPIRATION IN FISHES. 369 



ingly simple inversion ; such as that which forms the respiratory sac of 

 the Leech, having the blood-vessels distributed upon its walls. A 

 higher form of the gill, such as is found in Fishes and in the higher 

 aquatic Invertebrata, is seen at c, the surface being greatly extended, 

 by subdivision into minute filaments. A more complex form of the 

 pulmonary apparatus, such as is found in the higher Vertebrata, is 

 shown at d, the blood being distributed, not merely to its outer walls, 

 but to the minute partitions which subdivide its cavity into cells. And 

 at e is represented the respiratory organ of the Spider-tribe, which 

 bears an obvious resemblance to the lung of the Vertebrated animal, 

 shown at d; whilst it is evidently as nearly allied to the gill shown 

 at c, provided this be imagined to be sunk within a cavity formed by a 

 depression of the external surface, instead of projecting beyond it. 

 Thus we see how very close is the real resemblance between all the 

 forms of the respiratory apparatus, however unlike each other they may 

 at first sight appear to be. 



663. The gills of Fishes correspond with those of the higher Mollusca 

 in all essential particulars ; but they are more largely developed in pro- 

 portion to the size of the body ; and they are placed in a situation, that 

 enables them to receive a more regular and constantly-changed supply, 



Fig. 104. 



Capillary network of a pair of leaflets of the gills of the Eel : a, a, branches of the branchial artery con- 

 veying venous blood; b, b, branches of the branchial vein, returning aerated blood. The disappearance of 

 the dark shading in the network, as it traverses the gill, is designed to indicate the change in the character 

 of the blood, as it passes from one side to the other. 



both of blood and water. The gills are suspended to bony or cartilagi- 

 nous arches, of which three, four, or more, are fixed on either side of 

 the neck ; and the fringes hang loosely within a cavity, which commu- 

 nicates on the one hand with the mouth, and on the other with the ex- 

 terior of the body. The mechanism of respiration is very complex in 

 these animals ; and is evidently adapted to produce the most effectual 

 aeration possible. The mouth is first distended with water ; and its 

 muscles are then thrown into contraction, in such a manner as to expel 

 the water, through the aperture on either side of the pharynx, into the 

 gill-cavity. At the same time, the bony arches are lifted and separated 

 from each other, by the action of muscles especially adapted to this 

 purpose ; so that the gill-fringes may hang freely, and may present no 

 obstacle to the flow of the water between them. When they have been 

 thus bathed with the aerating liquid, and their blood has undergone the 

 necessary change, the water is expelled through the outward aperture 

 on each side of the back of the neck ; which is furnished with a large 

 flap or valvular cover, termed the operculum. In some of the cartila- 



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