674 



ICHTHYOLOGY. 



Structure 



and 



Functions 

 (if Kishes. 



SECT. VI. Organs of Respiration. 



Gill-lid. 



,Gill-6ap. 



Gill-open- 

 ing. 



As the organs of respiration appear in fishes under 

 a new form, very different from the lungs of the higher 

 order of animals, they demand our attentive considera- 

 tion. Many quadrupeds, birds, and reptiles, reside in 

 water, but nre obliged to come to the surface frequent- 

 ly in order to respire. But as fishes live immersed in 

 the water, they arc furnished with certain organs called 

 Gills, instead of lungs, to enable them to exercise the 

 functions of respiration in the fluid in which they re- 

 side. In many of the inferior animals, respiration is 

 performed by the same apparatus ; but as it appears in 

 its most perfect form in fishes, its examination will be 

 the move interesting. 



These organs of respiration in fishes consist of four 

 parts, a gill-lid, a gill-flap, the gill-opening, and the 

 gills themselves. 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, be- 

 ginning with those which are exterior. 



1. Gilt-lid. The gill-lid, or as it is also termed, oper- 

 culum, is situated behind the eye on each side. It is 

 scaly, membranaceous, or bony, and is articulated to the 

 bones of the head. It consists sometimes of one piece, 

 or of two or more, and is therefore termed monophyl- 

 lous, diphyllous, or triphyllous. 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 ab- 

 sent in fishes which have fixed branchiEe, and in a few 

 with free branchias. When it does exist, the characters 

 which it exhibits in its structure are subject to little 

 variation, and have been employed by La Cepede in the 

 construction of his orders. 



2. Gill-jlap. This is the membrana branchioslcga of 

 Linnseus, and was considered by him as a true fin. It 

 consists of a definite number of curved bones or carti- 

 lages, with a membrane. Its posterior edge is general- 

 ly 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 osseous fishes. When present, it ap- 

 pears to assist the mouth in promoting the current of 

 water through the gills, or perhaps forms a current 

 over the gills when the mouth is occupied in seizing prey. 



The gill-flap furnishes to the systematic ichthyologist 

 some of his most useful characters. He seldom pays 

 attention to its form, but its rays are eagerly counted, 

 as he finds that they are not subject to vary. Spe- 

 cies of the same genus have, in general, the same num- 

 ber of rays, and many of the Linnaean genera depend 

 on this circumstance for their character. Artedi, on 

 this subject, draws the following conclusion : " Quod 

 numerus ossiculorum in membrana branchiostega pri- 

 mum et prsecipuum characterem in distinguendis ge- 

 neribus piscium catheturorum et osteopterygiorum sup- 

 peditet." But in counting their number, care must be 

 taken to examine the structure of the gill-lid at the 

 same time, as the student sometimes enumerates among 

 the rays of the gill-flap the posterior divisions of that 

 organ, when present, and hence finds his observations 

 at variance with the descriptions of authors. 



3. Gill-opening. This division of the organs of re- 

 spiration presents many remarkable differences. In the 

 osseous fishes, and among the branchiostegi, this open- 



5 



ing is a simple aperture behind the gills on each. It is 

 sometimes round, or semi-lunar, and in relative posi- 

 tion it differs according to the species or genera. In 

 the cartilaginous fishes, the opening on each side is 

 subdivided into as many apertures as there are gills, 

 the gills in this tribe being fixed to the membranes 

 which act as partitions in the opening. In such fishes, 

 these openings arc on the summit, at the sides, or under- 

 neath, according to the genera. 



4. Gills. In the fishes with gills or branchia;, these Gills, 

 organs are in general eight in number, four on each 

 side. Each gill consists of three parts, a cartilaginous 

 or bony support, and its convex and concave sides. 

 The support of each gill consists of a crooked bone or 

 cartilage, in general furnished with a joint. At its 

 base, it is united with the bones of the tongue, and 

 above with those of the head. At both extremities it 

 is moveable, and throughout is flexible like a rib. Its 

 position is nearly vertical. From its exterior or con- 

 vex side, issue a multitude of fleshy leaves, or fringed 

 vascular fibrils, resembling plumes, and closely con- 

 nected at the base. These are of a red colour in al- 

 most all fishes in a healthy state. The internal or con- 

 vex side of the support next the mouth exhibits many 

 singular differences. It is always more or less furnish- 

 ed with tubercles. These in the genus Cyprinus are 

 smooth in the Cottus rough. They are lengthened 

 into slender spines in the herring and smelt, but in the 

 former these are serrated, while in the latter they are 

 smooth. This concave part of the gill is of a white 

 colour, and forms a striking contrast with the colour 

 of the convex side. 



In some osseous fishes, the number of gills exceeds 

 four on each. In the herring, for example, there is a 

 small imperfect gill on each side attached to the inner 

 side of the gill-lid, on which all its motions depend. 

 It has no bony arch nor concave side. At the entrance 

 to the gullet, there is a cartilage on each side, studded 

 with tubercles, resembling in appearance the concave 

 side of the last gill. In the plaise, a similar gill may 

 be observed on the inside of the gill-lid, but no distinct 

 appearance of a sixth gill at the entrance to the oeso- 

 phagus. 



In the chondropterygii, the gills are far from being 

 so perfect. They are fixed to partitions which serve 

 the purposes of the bony arches in the osseous fishes. 

 These partitions extend from the mouth to the gill- 

 opening, and vary in number according to the genera. 

 They are destitute of the inner or concave white side, 

 but the fleshy leaflets are of the same structure with 

 those on the convex part of the gills in osseous fishes. 



We are indebted to that acute anatomist, Sir Everanl 

 Home, for some important observations on the respi- 

 ratory organs of the lamprey and myxine, the apodal 

 chonropterygious fishes, and the least perfect in the 

 system. "In the lamprey, (he says), the organs of 

 respiration have seven external openings on each side 

 of the animal ; these lead into the same number of se- 

 parate oval bags, placed horizontally, the inner mem- 

 brane of which is constructed like that of the gills in 

 fishes. There is an equal number of internal openings 

 leading into a tube, the lower end of which is closed, 

 and the upper terminates by a fringed edge in the oeso- 

 phagus. These bags are contained in separate cavities, 

 and enclosed in a thorax resembling that of land ani- 

 mals, only composed of cartilages instead of ribs, and 

 the pericardium, which is also cartilaginous, is fitted to 

 its lower extremity like a diaphragm." In the myxine, 



Structure 



and 



Functions 

 oi h'i-hes. 



