SCIENTIFIC SURVEY. 



41 



Fife 2. 



Gill lid, (a) — This is 

 called the preoper- 

 cle or preoperculum, 

 and is situate behind 

 the eye on each side. 

 In its structure it is 

 scaly, membranace- 

 ous or bony, and is 

 articulated to the 

 bones of the head. It consists of one or more pieces, and is 

 therefore termed monophyllous when but one, diphyllous when 

 two, triphyllous three, and so on. Its use is to support the gill 

 flaps and act as cover to the opening of the lids. It is absent in 

 fishes which have fixed branchias, and in a few with free bran- 

 chi^. 



Gill flap, (6) — This was called by Linneus membrana hrancMostega 

 (branchiostegal membrane, or gill covering membrane.) He (Lin- 

 neus) also considered it a true fin. It is also called opercle or 

 operculum. It consists of a number of curved bones, or carti- 

 lages with a membrane. Its posterior or hindmost edge is gen- 

 erally free, and its anterior edge or base is united with the gill 

 lid. (a) It is capable of extension and contraction, and when at 

 rest is folded up partly under or beneath the gill lid (opercle). 

 In some fishes it is wanting. When present it appears to assist 

 the mouth in promoting the current of water over the gills. The 

 rays of this organ (c. d. e.) sometimes called Branciostegal rays, 

 furnish the Ichthyologist with valuable distinctive characters. 



Gill opening — In the osseous (bony) fishes, and among the Bran- 

 chiostegi (of some authors) this opening is a simple aperture 

 behind the gills on each side formed by the lifting up the gilljlap. 

 It is sometimes round or semilunar, and in relative position dif- 

 fers according to the genera and species. In cartilaginous fishes 

 as the "Lamper Eel," for instance, the gill opening is a round 

 hole or aperture over each gill. 



Gills (proper) — Lifting up the gill flap you will see directly be- 

 neath them the true gills, several organs consisting, as has been 

 before said, of a multitude of red colored, fringed vascular fibrils, 

 or delicate fleshy plumes attached to curved bones jointed on to 

 the head. In fishes with gills, or branchiae as they are often 



6 



