Mar. 26, 1 8 74 J 



NA TURE 



407 



roughly established, and thus the probability of the exist- 

 ence of lungs within the class of fishes is also established. 



But what is a luni;? 



A lung is a sac-like structure capable of being distended 

 with air, supplied with venous blood direct from the heart 

 and sending arterial blood directly to it. Generally the 

 whole of the blood from the lungs goes back to the heart 

 directly, but in one Batrachian — the celebrated Proteus — ■ 

 a portion of the blood from the luni;s finds its way not 



Fig. Si. 



Fig. 80.— The Circul.ation in a Tadpole at a more advanced stage, when the 

 gills have begun to be absorb=d, the pulmonary arteries to increase, as 

 also the connecting branches (at the root of the gills) between the 

 branchial arteries and branchial veins. 



Fig. 8i.— The Circulation in a young Frog. Here the gills have been ab- 

 sorbed, and the blood passes directly from the heart to the head, the 

 dorsal aorta, the lungs, and the skin, 



into the heart but into vessels of the general circulation. 

 When there is an air-sac which does not both receive 

 blood directly from and return it directly to the heart — 

 i.e. when there is no \xixe. pulDionatv circulation — such an 

 air-sac (whether single or double) is termed a sivim- 

 bladiier and a structure of the kind is found in very many 

 fishes. The swim-bladder of ordinary fishes neither re- 

 ceives blood directly from the heart by an artery like the 

 pulmonary artery of higher animals, nor does it return 

 blood directly to the heart. 



The transition, however, from a lung to a swim-bladder 

 is a graduated one. We have just seen that in Proteus, 





/— ' 



Fig. 82. Fig. 83. 



Fig. 82. — Diagram representing the main arteries of a Bird (.''owl) with the 

 changes induced on the primitive condition (after H. Rathke). a, a, 

 internal carotids ; h, b. external carotids ; c. c. common carotids : d, root 

 of main aortic arch (here right); c, arch of the same ;/, right subclavian 

 (which arises from the anastomosis of the first two right primitive aortic 

 arches) ; ^, commencement of the descending aorta ; ^, /r, left subclavian ; 

 i, /, r", pulmonary arteries : k. right, and I. left, rudiments of the primi- 

 tive aortic arches. 



Fig. 83.— Di.agram representing the main arteriei of a Mammal with the 

 changes induced in the primitive condition (a'ter H. Rathke). a, l\ c, 

 carotids, as before ; d. root of main aortic arch (here left) ; e, arch of 

 the same ; J", commencement of descending aorta ; ff, left vertebral 

 artery ; /i, left subclavuin ; i, right subclavian ; *. right vertebral 

 artery: /, conlinuation of right subclavian; m, pulmonary artery: n, 

 remnant o! left primitive aortic arch. 



though blood is returned from the lungs direct to the 

 heart, yet that not all the blood is so returned. On the 

 other hand in another animal, Ceratodus, though blood 

 is not brought to its air-sac directly (which is therefore a 

 swim-bladder and not a lung), yet for all that blood is 

 ^ent from it direct to the heart. 



Ceratodus (or as it is locally called " flat-head ") is a fish 

 of Queensland, closely allied xaLepiaosiiCH, and is a very 



noteworthy animal apart from and in addition to its pecu- 

 liarly transitional structure as regards its air-sac. 



It is, indeed, the last of an ancient race, a species 

 of the same genus (known almost exclusively by its 

 teeth) being found fossil in strata of oolitic and triassic 

 date. It was discovered by the Hon. W. Foster, M.C.A. 

 Mr. Gerard Krefft, F.L.S., Curator and Secretary of the 

 Sydney Museum, first described and figured the animal in 

 1870,* and at once correctly referred it to the genus Cera- 

 todus, which up to that time was supposed to be entirely 

 extinct. Its further determination was effected by Dr. 

 Giinther.f He has conclusively shown that Ceratodus and 

 Lepidosiren are closely allied, and thus finally brought the 

 latter definitively within the class of Fishes, for that Cera- 

 todus is a fish no one questions. It is an animal, how- 

 ever, of somewhat amphibious habits, as at night it leaves 

 the brackish streams it inhabits, and wanders amongst 

 the reeds and rushes of the adjacent flats. Vegetable 

 substances constitute its principal food. 



Ceratodus and Leptdos/re/i together afford the most re- 

 markable evidence of the persistence of the same type of 

 structure in the Vertebrate sub-kingdom. The group to 

 which they both belong reaches back into the very earliest 

 epoch, which has yet aftbrded us any evidence wh.atever 

 of the existence of fishes ; while the genus Ceratodus 

 seems to have persisted unchanged from the period of 

 the deposition of the triassic strata. 



Suz/imary. 



Taking a rapid retrospect of the course we have pur- 

 sued, we find that in seeking to decide as to " What is a 

 Frog.'" our inquiry into its absolute structure has made 

 known to us an animal of peculiarly specialised and per- 

 fect organisation. This has been shown to us pre-eminently 

 by the study of its skeleton. We have especially noted its 

 skull, its wonderfully short vertebral column, its utterly 

 anomalous pelvis, and its scarcely less anomalous foot. 

 The flesh which clothes that skeleton has been seen to 

 exhibit distinct muscles wonderfully like our own, those 

 of the foot, indeed, exceeding ours in number, and being a 

 very marvel of complexity. We have met with a nervous 

 system ministered to by delicate organs of sense, and 

 noted for the ready response to stimuli, made by even 

 separated parts of it as evidenced by strikingly co- 

 ordinated complex movements. We have found the 

 circulation to be carried on by a heart which, at first 

 sight, seems too structurally imperfect to distribute 

 the venous and arterial blool in their respectively ap- 

 propriate channels. Nevertheless, further examination 

 has shown us that this heart is provided with a special 

 arrangement of parts so delicately co-adjusted as to be 

 able to act thus as efficaciously as does the heart of ani- 

 mals much higher in the scale. Respiration, too, we have 

 seen provided for partly by an effective throat air-pump, 

 partly by a peculiar activity of the cutaneous structures. 



We have, moreover, found that tliis complex adult 

 condition is arrived at by means of a rapid metamor- 

 phosis from an immature condition wonderfully different, 

 indeed, but no less perfectly adapted to the life con- 

 ditions of the tadpole state. 



It remains now " to sum up the results" of our inves- 

 tigations through "a series of wider and wider com- 

 parisons" to arswer, finally, as far as may be, the initial 

 question of this little treatise. 



We have, in the first place, seen that the frog belongs 

 to an order far more distinct from cognate ordinal groups 

 than is man's order from other orders of his class mam- 

 malia. We have also seen that the frog belongs to an 

 order which is singularly homogeneous, and yet that the 

 class which includes it is remarkably heterogeneous. 



Again, we have found that the subordinate groups of 

 the frog's order, families and genera, have very definite 



