INTR OD UC TION. xvii 



which winds in a corkscrew fashion from the pylorus to the anus ; by these means of course 

 the absorbing surface is considerably increased ; it is generally thick and glandular, always 

 vascular ; it is reticulate in many fishes, as in the Mursena and the Sturgeon ; in the Salmon 

 the mucous membrane is more or less rugose. 



The liver in fishes is generally of a large size and well developed, and frequently 

 contains an enormous quantity of oil, this alone forming an important article of commerce ; 

 its texture is usually very soft ; the lobes are often numerous ; a gall-bladder, from which 

 the bile is poured into the intestine through a single duct, which terminates near the 

 pylorus (see fig. f), is, as a rule, present, though there are a few exceptions. Among fresh- 

 water fishes, the gall-bladder is absent in the Lampreys. 



In fishes the kidneys consist of two lengthened dark red-brown bodies, on each side of 

 the median line of the body, beneath the vertebrae, extending through the whole or the 

 greater part of the dorsal region of the abdomen. The kidneys of a fish are readily dis- 

 cernible ; they form that long red band which lies adjacent to the backbone, easily seen 

 after the extraction of the other viscera ; it is that part of a fish which a careless cook fails 

 to clean out thoroughly, which can only be done by means of several scrapings with the 

 sharp point of a knife and copious ablutions of pure cold water. 



The respiration of all fishes is purely aquatic ; it is beautifully effected in all osseous 

 fishes, with the exception of the Lophobrauchii, — as the Pipe-fishes iSyngnathus), and the Sea- 

 horses (Hippocampus), — by means of gills or branchice. These organs consist of a single or 

 double series of flat cartilaginous bodies which support delicate fringes richly supplied with 

 blood ; above, the gills are united to the under side of the head ; below, they are connected 

 with the tongue or hyoid bone. The mechanism of the respiratory process is simple in 

 nearly all osseous fishes. The water is taken in at the mouth, and bathes the branchial 

 fissures; having lost its oxygen, the water is forcibly driven through the wide opening on 

 each side of the neck called the gill-fissure, which fissure is closed in front by a series of 

 flat bony scales called the gill-cover or "operculum." The normal number of these vascular 

 branchial arches is four on each side of the hyoid bone in osseous fishes ; in cartilaginous 

 fishes the usual number is five ; in the Cyclostomata, as in the Lampreys, it is seven ; in 

 these last-named fish the oxygenating water does not reach the branchial sacs through the 

 mouth ; it passes through the gill-sac openings by a tube leading into the pharynx, from 

 whence it passes into the gill-sacs, these gill-sacs freely communicating with each other 

 through the pharynx. 



The smaller the external orifice of the gills, and the closer the gill-cover fits on these 

 organs, the greater is the power in the fish to bear exposure to the outward air; so long 

 as the branchial laminae are kept moist, they can, to some extent, perform their function of 

 appropriating to themselves the oxygen of the water retained within the branchial chamber; 

 but when the external aperture of the gills is large, desiccation by the atmospheric air 

 takes place, and the delicate branchial laminae collapse, and death speedily ensues, for the 

 blood can no longer effect a passage through them. Perhaps of all fresh-water fishes the 

 Eel is best able to survive a lengthened period out of its watery element. In the Eels the 

 gill-opening is a mere external fissure, a small vertical slit, and is removed very far back ; 

 so that the cavity, which lodges the branchiae, is converted into a long chamber wherein can 

 be retained a considerable quantity of water; so that in early summer mornings, when the 

 dew is on the grass. Eels are able to make their overland-way to some distance from one 

 piece of water to another. 



The Auabas, or Climbing Perch of the tropics, has receptacles in which it can retain 

 water, as in reservoirs, wherewith to moisten the folded branchial laminae. Besides the 

 branchiae, most osseous fishes possess certain vascular bodies called "pseudobranchiae." There 

 are genera in which these organs have not been detected ; they are situated on each side 



