February, 1907.'' 



KNOWLEDGE & SCIENTIFIC NEWS. 



35 



A Fish Out of Water. 



By Felix Oswald, D.Sc. 



The well-worn saying, " to feci like a fish out of water," 

 obviously embodies the general belief that fishes are 

 absolutely confined to the watery elei.ent. Nature, 

 however, never allows herself to be cramped by hard 

 and fast rules deduced from insufficient observation; 

 for, although a typical fish is eminently adapted for 

 swimming and breathing under water, yet tl.ere are 

 several striking deviations from this normal state of 

 things. For instance, the hopping gobies (in which 

 the fore-fin has developed a distinct elbow-joint) can 

 leave the sea and habitually skip along the shore in pur- 

 suit of insects and molluscs; the climbing perch can 

 exist for days out of water, and is even said to climb 

 palm trees, whilst the aerial flights of the flying fish are 

 known to all. These exceptions to the ordinary habits of 

 fishes are not merely of individual interest, but help us 

 materially to realise the somewhat analogous, but more 

 successful, struggle to invade the land which occurred 

 long ago, in pre-Carboniferous ages, on the part of far 

 less highly specialised fishes. 



The radical organic changes resulting from this 

 invasion of a different element were mainly two-fold; 

 firstly,' in respiration — breathing by gills being ex- 

 changed for breathing by lungs, and secondly, in loco- 

 motion — fins being superseded by five-fingered, jointed 

 limbs. 



Fig. I.— The Hopping: Ooby IPeriophthalmus), 



In the modern invaders, such as the climbing perch 

 {Anabas scandcns) and the h(jpping gobies {Pcriophthal- 

 mus and Boleo phthalmus), gills still continue to be the 

 seat of respiration, but their structure is much modified. 

 In the case of the hopping gobies (Fig. i) the only 

 change in structure of the breathing-organs is an in- 

 crease in the size of the gill-cavity, which is thereby 

 able to contain air as well as water. The gills, how- 

 ever, are much reduced; and respiration seems also to 

 be carried on by the thin skin of the tail-fin. A more 

 advanced state of things occurs in the climbing perch, 

 in which there is an accessory organ in the gill-cavity, 

 consisting of labyrinthine folds of mucous membrane 

 (Fig. 2), so as to expose a larger respiratory surface to 

 the air — a direct result of terrestrial conditions. This 

 organ functions as a lung, by means of which the fish 

 is able to exist out of water for a long time, a fact 

 which Indian jugglers have made use of in adding this 

 fish to their stock-in-trade. 



These labyrinthine organs may be profitably com- 

 pared with the somewhat analogous lung-like out- 

 growths in the upper half of the gill-cavity in land- 

 crabs (Gccarcinm and Birgiis lairo). 



The lungs of land-vertebrates have, however, laeen 

 derived, not from the gills, but from the air-bladder of 

 fishes. This theorv is based essentiallv on the facts of 

 development, and is now generally admitted. Both 

 lungs and air-bladder arc formed bv an outgrowth from 

 the gullet, but the lungs arise ventrallv. while the air- 



bladder in most fishes has a dorsal origin. This differ- 

 ence in position is not, however, so serious an objection 

 to the theory as it would appear at first sight, for in 

 Erythrinus it arises laterally, in Ceraiodus it becomes 



Fig. 2.— The Climbing Perch iAna^ai ^cawUm,. The lower figure display* 

 the gill-chamber; a, the labyrinthine supra-branchial organ; 

 6 the gills. 



I'ig. 3. — Diagrammatic transver.ie and longitudinal sections miter 

 Deam of air-bladder (il and adjacent digestive tract (■') of A, 

 Sturgeon and many Teleosts iPhyso.itomi' ; '>', »yi)iriiiii» ; C, 

 Ceratodm ; D, Lrpiilotirtn and ProlopUrui. 



more ventral in position, and, finally, in Polypterus, 

 Calaiiioiclitliys, r.cpiJosirai, and Protopt<rus. it has 

 actually the same median ventral position as in all true 

 lungs (Fig. 3). 



Now, when one organ is evolved from another, tliere 



