870 SPECIAL PHYSIOLOGY. 



propelled through the gills, before it is again distributed to the body ; never- 

 theless, the respiration of fishes, being aquatic, is feeble, and their tempera- 

 ture is cold. The gills of fishes are not ciliated on their surface ; but it is nec- 

 essary that they shquld continue moist, or the usual respiratory interchanges 

 between the blood and the air dissolved in the water, would soon cease. Res- 

 piration will, however, go on for a short time in the air, provided that the 

 gills remain moist. Certain fishes, as the eel and others, leave the water for 

 a time. The Anabas, of Ceylon, is said even to climb up trees and bushes 

 after food; on the sides of its head, just above the gills, the pharyngeal bones 

 are convoluted, and the anterior branchial arches support chambers with lami- 

 nated walls, for holding quantities of water. Fishes are necessarily suffocated 

 when removed from the water, the gills becoming first clogged and then dry, 

 circulation and respiration in them being both arrested. They are also as- 

 phyxiated in water which no longer contains oxygen in solution, or even when 

 foreign substances are dissolved in it; sugar speedily destroys them. In that 

 exceptionally organized marine animal, the Amphioxus, the mouth is provided 

 with ciliated lobes., which, by ciliary action, propel the water into the pharynx. 



This cavity is dilated, has its sides supported by a complex lattice-work of 

 branchial cartilages, upon which the branchial vessels are placed; its sides 

 are perforated by numerous slits, upwards of 100 in number, the edges of 

 which are ciliated, and which lead into the perivisceral cavity of the abdomen. 

 The water used for respiratory purposes is expelled from this cavity by an 

 opening named the abdominal pore. Moreover, the entire alimentary canal 

 is ciliated internally, and the water which passes through it may also be 

 employed in the aeration of the blood. 



On comparing the entire series of the Vertebrata, as regards their mode of 

 respiration, it is seen that, in the adult state, the Mammalia, Birds, Reptiles, 

 and Caducibranchiate Amphibia, breathe by lungs; that the Perennibran- 

 chiate Amphibia, and the tadpoles of the Caducibranchiate kinds, breathe 

 both by lungs and gills; and, lastly, if we disregard the sub-respiratory air- 

 bladder occasionally present, that Fishes breathe entirely by gills. The respi- 

 rator^ process in these animals is, however, much more active than in the 

 aquatic Non-vertebrata, which we have next to describe. This is due to the 

 comparatively greater size of the gills, to their more perfect and complex struc- 

 ture, to the special contrivances for moving the water over their surfaces, and 

 to the proximity of the heart to these organs. 



Mollusca. Of these chiefly aquatic animals, the most highly developed gills 

 exist in the highest Class, the Cephalopods, in which the branchiae, being very 

 large, but nonciliated, consist of foliated laminae, united by a common stem, 

 which supports the branches of the branchial arteries and veins. The gills 

 are lodged, one or two on each side, in the cavity of the nonciliated mantle, 

 which has two orifices, one by the side of the neck, at which the water enters, 

 and another placed at the extremity of the tubular process, called the funnel, 

 from which the water passes out. The movement of the water is accom- 

 plished by the alternate dilatation and contraction of the muscular walls of the 

 mantle. In the Pteropods, the branchiae are also laminated, being placed 

 sometimes within, and sometimes without, the mantle, and being always cili- 

 ated. In the naked species of Branchio-gasteropods, the gills consist of cili- 

 ated, fringed, sometimes tubular processes, projecting into the water (Nudi- 

 branchiata), and arranged either along the sides of the body in tufts (Tritonia), 

 or else in a circular disc-like manner on the dorsal region (Dorsibranchiata), or 

 around the lower opening of the alimentary canal (Doris); sometimes a fold 

 of the mantle partially or completely covers them. In those kinds which have 

 univalved shells, the gills consist of ciliated plicated fringes, lodged in the last 

 spire of the shell, the water gaining access to the cavity in which they are 

 contained, sometimes by a long tube, and sometimes by a wide opening. 

 Amongst the lowest branchio-gasteropods are some species allied to the Nudi- 

 branchiate group, which have no branchiae, but are believed to respire by 

 means of their surface only, or by that, aided by certain ciliated extensions of 

 the digestive cavity. A few genera, such as Onchidium, possess, besides arbo- 

 'rescent branchiae, distinct air-sacs, showing a transition between the branchi- 

 ate and pulmonate divisions of the Gasteropoda. In the bivalved Lamelli- 



