I894-] 



THE BRITISH NATURALIST. 



65 



between the blood and the water is effected ; in other words, the hag 

 has on either side seven gill-sacs ; each sac being separate and 

 distinct. In the lampreys we find a somewhat higher type; the 

 sacs are distinct, but are relatively larger, and instead of com- 

 municating with the pharynx and exterior by tubes, open by little orifices 

 directly. It is to be noticed that the sacs are more flattened and much 

 nearer together than in the hags. In all the suctorial fishes the water is 

 both admitted and expelled through the fourteen gill-orifices when the 

 animal is adhering to any object, animate or inanimate, with its mouth. 

 Passing to a higher group of fishes, the Elasmobranchii (sharks and rays), 

 we find the gill-sacs still more flattened antero-posteriorly ; the com- 

 munication between the sacs and the exterior being through slit-like 

 orifices, not circular ones. The septa, or partitions between the sacs, are 

 very slight, and the lining membrane is raised into folds. In the highest 

 member of this group (chimasra) we find the first trace of the gill cover. 

 Passing to higher groups we find the septa, which internally are sup- 

 ported by the branchial arches, gradually diminishing in outward length ; 

 whilst the membranous folds extend as far as previously, so that they 

 reach beyond the septa ; with the result, that instead of a septum or sac 

 wall reaching from each branchial arch to the exterior wall of the body, 

 and supporting for a part or even the whole of that distance distinct 

 membranou folds, we have the membranous folds developed into free 

 gills, whose only external protection is the operculum or gill-cover. In 

 the highest fishes, if the gill-cover be removed, each branchial arch with 

 its free gill looks like a comb ; but a closer inspection of what at a casual 

 glance resemble the teeth of a comb shows that there are two rows of 

 teeth or gill-plates, not paired, but alternating, and that each is a gill or gill- 

 plate in whose minute blood vessels the aeration of the blood is effected. 

 In some fishes even the lamellae, as the gill-plates are called, tend to be still 

 further broken up and offer an increased respiratory area. It will be obvious 

 that the gill-plates represent the membranous folds of the lower fishes. 

 The young of many fishes possess external gills resembling in structure 

 those of the perennibranchiate batrachians, or those possessed by the tad- 

 poles of frogs and newts. Similar gills, as well as internal ones and 

 lungs, are possessed by those very remarkable fishes, the Dipnoi, of which 

 there are only three known existing species. If we only examine the 

 breathing apparatus of the existing icthyopsidans we shall find many 

 difficulties in the way of the doctrine of descent, but when we examine 

 the fossil forms many of those difficulties vanish, and there can be no 

 doubt that a fuller knowledge of the fauna of the carboniferous period 

 (to say nothing of the others) would remove the residue of these difficulties, 

 which after all should scarcely create surprise if we remember the 

 enormous lapse of time since the deposition of the coal beds, at which 

 period, millions of years ago, fishes and batrachians, almost if not quite 



