HARDWICKE'S SCJENCE-G OSS/P. 



57 



those creatures which exist in water ; and indeed, 

 taken in this its widest sense, it is scarcely to be seen 

 why the authorities have made no efilbrt to introduce 

 other classes among the invertebrates, one of which is 

 notably absent, namely, the molluscs. There would 

 scarcely be anything more interesting than a series of 

 fanks in which were to be seen crawling about on the 

 rocky bed, or over the sandy floor, the inhabited shells 

 of those creatures which we are accustomed to see 

 lying in cabinets and on mantel-shelves, artificially 

 polished, and in many cases in sharp contrast to their 

 dull natural appearance. 



Peihaps one of the most interesting tanks is that 

 devoted to the beautiful Guillemots, or swimming birds, 

 although few visitors have the opportunity of seeing 

 ihem at their best. This is at their feeding time, 

 when they exhibit their wonderful powers of diving. 

 When the surface of the water, as seen from below, 

 is perturbed by the almost phosphorescent wavelets, 

 caused by the birds splashing about and cleaving the 

 water at full speed in pursuit of their prey, the 

 sudden transformation of a bird into a fish — for such 

 it almost appears in the water — is a most striking 

 sight, and as it cuts through the surface with its beak, 

 and folds its powerful wings by its side when gliding 

 through the water with the impetus it has gained, it 

 sliines with a silver-like glow, as it reflects the rays 

 which illumine it from above. It seems principally 

 to use its legs in its under-water propulsion, its tail 

 doubtless acting the same part as that it plays in true 

 fishes. The bird has been said to remain beneath the 

 surface for several minutes. 



The strange lazy mud-fish in its table aquarium 

 scarcely perhaps attracts the attention it deserves, 

 and yet its life-history is a most important one to the 

 evolutionist, since it is one of those animals which 

 supply a found — and not a missing — link between 

 the Reptilia and the Pisces. In reptiles, the process 

 fjf breathing is carried on by means of well-developed 

 iungs, whilst in the fishes proper, the process of 

 oxygenisation of the blood is brought about by gills, 

 situated on both sides of the head behind the mouth. 

 It is scarcely necessary to repeat the fact that fishes 

 require air just at much as human beings, and that 

 if placed in water which has been boiled (and of 

 course cooled), they cannot live, or that if placed in 

 insufficiently aerated water they can often be seen 

 breathing the air at the surface of the aquarium. In 

 our friend the mud-fish, Lepidosiren or Protopterus, 

 however, there is not only the usual complement of 

 gills as in fishes, but also lungs, as in reptiles, the 

 ordinary swimming-bladder of fishes being in this 

 instance organised as a lung. The happy possession, 

 therefore, of both ,of these forms of breathing 

 apparatus, enables it to inhale air both directly 

 from the atmosphere, and by abstracting it from the 

 w-ater. In its native haunts it is found inhabiting 

 the rivers en the west coast of Africa. These at 

 certain seasons run quite dry. At such times, when 



it feels the stream gradually subsiding in which it has 

 dwelt, and the danger threatens of being stranded 

 I and exposed to the attacks of its enemies, it has the 

 habit of burrowing into the soft clay forming the bed 

 of the stream, and of there hiding itself in the nest it 

 has formed. As soon as the water has ceased to flow 

 over its place of refuge it commences to breathe by 

 means of its lungs, and remains ensconced in its 

 clayey home, until, with the return of the wet season, 

 the stream again fills up its deserted bed. By taking 

 advantage of this nidifying propensity, the fish was 

 brought to England in the clay in which it had 

 buried itself, and the nest is now to be seen by the 

 side of the aquarium in which the creature lives. It 

 would seem, too, as if it has resumed its fish-like 

 habits permanently, as no provision appears to have 

 been made for it in its confined home, by which it 

 can at all make use of that important organ, its 

 lungs. This is rather to be regretted, as to the 

 general public the novel sight of a comparatively 

 unknown fish living out of water, on a dry soil, 

 would have proved no doubt interesting and enter- 

 taining. 



Fishes, fossil and recent, are sometimes roughly 

 classified into two divisions according to the shape 

 of their tails. Agassiz, the great naturalist, whose 

 authority on the subject is everywhere recognised, 

 found that some tails were equal-lobed, as in the 

 case of the herring, whilst others, as those of the 

 shark, the skate, and the sturgeon, were unequal- 

 lobed, and consisted of an elongated upper lobe, into 

 which the backbone was continued, the lower lobe 

 being considerably shortened. It is an interesting 

 fact that, although now but very few living fishes 

 have tails of the unequal-lobed form, almost all of 

 the forms of primitive fish-life bore them. During a 

 period preceding that when the chalk was formed, 

 fish with equal-lobed tails commenced to live, whilst 

 the ancient form began to die out. The proportion 

 of one form to the other now, therefore, is reversed, 

 whereas homocercal (equal) tails were formerly the 

 exception, and heterocercal (unequal) tails the rule, 

 now, with the exception of the sturgeon, shark, 

 skate, bony-pike, and perhaps a few others, the far 

 larger proportion are equal-lobed. 



The little gar-pike, or bony-pike of the American 

 rivers, which are now in the Aquarium, are the first of 

 their species which have been introduced alive into 

 England, To the energy of Mr. Crane, F.G.S., and 

 his American friends, the authorities are greatly 

 indebted in this matter. The gar-pike exhibit well 

 the ancient form of unequal lobed tail. They attain 

 a length of several feet, and their vertebral column is 

 more completely ossified than any living fish. Their 

 jaws form a long narrow snout, which is armed by a 

 double series of teeth. 



Every schoolboy who has lived in a district where 

 the chalk-hills form an important feature in the land- 

 scape, has found at some period or other numerous 



