GIL 



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GIL 



they have a vast number of fibrillae 

 standing out on each side of them 

 like a fringe, and very much re- 

 semble the vane of a feather. There 

 are, in most fishes, four gills on each 

 side, resting on an equal number of 

 arched portions of cartilage or bone, 

 connected with the os hyoides. In 

 some cartilaginous fishes there are 

 five gills on each side ; in the lam- 

 prey there are seven. The larger 

 Crustacea have their branchiae situ- 

 ated on the under side of their 

 body, not only in order to obtain 

 protection from the carapace, which 

 is folded over them, but also for 

 the sake of being attached to the 

 haunches of the feet, jaws, and 

 thoracic feet, and thus participating 

 in the movements of those organs. 

 They may be seen in the lobster 

 and in the crab, by raising the 

 lower edge of the carapace. 



In the greater number of mollusca 

 these important organs, although ex- 

 ternal with respect to the viscera, are 

 within the shell, and are generally 

 situated near its outer margin. They 

 are composed of parallel filaments, 

 arranged like the teeth of a fine 

 comb; and an opening exists in 

 the mouth for admitting the water 

 which is to act upon them. These 

 filaments appear, in many instances, 

 to have the power of producing 

 currents of water in their vicinity 

 by the action of minute cilia, sim- 

 ilar to those belonging to the ten- 

 tacula of many polypi, where the 

 same phenomenon is observable. In 

 the Acephala, or bivalve mollusca, 

 the gills are spread out, in the form 

 of lamina?, round the margin of the 

 shell, as is exemplified in the oyster 

 where it is commonly known by the 

 name of beard. The aerated water 

 is admitted through a fissure in the 

 mouth, and when it has performed 

 its office in respiration, is usually 

 expelled by a separate opening. 



All the sepise have their gills 

 enclosed in two lateral cavities, 



which communicate with a funnel- 

 shaped opening in the middle of 

 the neck, alternately receiving and 

 expelling the water by the muscular 

 action of its sides. The forms as- 

 sumed by the respiratory organs in 

 this class are almost infinitely di- 

 versified. In fishes the gills form 

 large organs, and the continuance 

 of their action is more essential to 

 life than it appears to be in any of 

 the inferior classes. When their 

 surfaces are minutely examined, 

 they are found to be covered with 

 innumerable minute processes, 

 crowded together like the pile of 

 velvet ; and on these are distributed 

 myriads of blood-vessels, spread 

 like a delicate net- work, over every 

 part of their surface. A large flap, 

 termed the operculum, extends 

 over the whole organ, defending it 

 from injury, and leaving below a 

 wide fissure for the escape of the 

 water, which has performed its 

 office in respiration. For this pur- 

 pose the water is taken in by the 

 mouth, and forced by the muscles 

 of the throat through the apertures 

 which lead to the branchial cavities; 

 in this action the branchial arteries 

 are brought forward and separated 

 to a certain distance from each 

 other, and the rush of water 

 through them unfolds, and separ- 

 ates, each of the thousand minute 

 filaments of the branchiae, so that 

 they all receive the full action of 

 that fluid as it passes by them. 

 When a fish is taken out of the 

 water, the animal vainly reiterates 

 its utmost efforts to raise the bran- 

 chiae, and relieve the sense of suffo- 

 cation it experiences in consequence 

 of the general collapse of the fila- 

 ments of those organs, which adhere 

 together in a mass, and can no 

 longer receive the vivifying influ- 

 ence of oxygen. "It has been 

 generally stated," says Dr. Roget, 

 "by physiologists, even of the 

 highest authority, that the princi- 



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