344 PHILOSOPHY 01 ZOOLOGY, 



which, at its separation from the heart, forms a bulb vary- 

 ing in shape according to the species. This artery sub- 

 divides and proceeds directly to the gills, over whose leaves 

 it is spread in the most minute ramifications. 



The organs of respiration in fishes consist of four parts, 

 n gill-lid, a gill-flap, the gill-opening, and the gills them- 

 selves. The two last are always present, but one, and 

 sometimes both, of the two first are wanting. We propose 

 to examine these parts in succession, beginning with those 

 which are exterior. 



1. Gill-lid. The gill-lid, or, as it is also termed, opercu- 

 lum, is situated behind the eye on each side. The bones 

 of which it consists vary in form and number. The first, 

 termed by CUVIER the pre-opercle, is united with the cheek- 

 bones, and extends, in the form of a thin plate, from the 

 occiput to the articulation of the lower jaw. To its distal 

 margin are attached the opercle, next the occiput ; then the 

 sub-opercle, and, last of all, the mter-opercle, next the 

 tongue. The surface in some is smooth, in others rough, 

 or tuberculated, or striated, or spinous. Its use is to give 

 support to the gill-flap, and act as a cover to the opening 

 of the gills. It is absent in fishes which have fixed 

 branchiae, and in a few with free branchiae. When it does 

 exist, the characters which it exhibits in its structure are 

 subject to little variation. 



The gill-flap is the membrana branchiostega of LIN- 

 N/EUS, and was considered by him as a true fin. It consists 

 of a definite number of curved bones or cartilages, with a 

 connecting membrane. Its posterior edge is generally free, 

 and its anterior edge or base is united with the gill-lid. It 

 is capable of extension and contraction, and, when at rest, 

 it is generally folded up under the gill-lid. It is wanting 

 in the chondropterygii, and likewise in a few genera of os- 

 seous fishes. When -present, it appears to assist the mouth 



