35 



KNOWLEDGE & SCIENTIFIC NEWS. 



IFliBRUARY, 1907. 



is usually cithor a concomitant change of fundion, or 

 else a subordinate function may become exalted by tiie 

 altered circumstances into the first place. 



The air-bladik'r in the m.ijority of cases has a jnuely 

 hydrostatic fum-lion, and swms t» prevent the fish 

 from too suddenly altering the depth at which the 

 conditions of life are most ad\ anlagcmis li> ll. l>nl il 

 the cli.inge in le\el be made grailuallx-, the necessary 

 compensation is oblaineil by the secretion or absorp- 

 tion of the gas in the air-bladder through the abundant 

 blood-capillaries (nV/i; iiiirahilKi), which usually line the 

 walls. For instance, Moreau has show 11 by his experi- 

 ments that if the air-bladder is emptied of the gas it 

 contains, the fish sinks to the bottom of the vessel in 

 which it is living, and cannot regain its ec|uilibrium 

 until it has secreted a fresh supply of gas. With regard 

 to this hvdrostatic function, it is interesting to note 

 that the air-bladder is not jjossessed by fiat-fishes, 

 living to the reason that they habitually rest on the 

 ground and lead a \ery sluggish existence. Hut il is 

 e(|ually absent in the large and ancient group ol 

 I';iasmobranchs (sharks, dog-lish, itc), unless llu' 

 obscure pharyngeal pouches should be regarded as 

 degenerate rudiments. Perhaps the notoriously pre- 

 datory habits of these fishes would ha\e rendered il 

 a disadvantage for them to have acquired an organ 

 w hich restricts a fish to a definite zone-level. 



.Although the secretion of gas is the chief function 

 of the air-bladder, yet (as already indicated) some 

 absorption can. and does, take place in those fishes in 

 which the walls of the bladder are lined by blood- 

 capillaries {rctia nurabilui).* The two processes of 

 .secretion and absorption have only to take place alter- 

 nately and rhythmically in order to be equivalent to the 

 expiration and absorption of a true lung. Of course, 

 any actual breathing by means of the air-bladder can 

 only take place in those fishes in which the duct be- 

 tween the throat and the air-bladder still remains open 

 (the Pliysoslomi). Tliis connection with the exterior 

 l>8Comes of the greatest importance to the species 

 whenever the water it inhabits becomes muddy or in- 

 sufficiently aerated. Thus many of the meml>ers of the 

 great fresh-water family of cyprinoids (bleak, carp, 

 etc.) need to come periodically tO' the surface to sw-allow 

 air, ow ing to the frequent paucity of oxygen in the still 

 and stagnant waters which they inhabit. Many 

 siluroids, too, may be buried in mud for a long time 

 during the dry season, or can also travel on land from 

 one lake to another. Even our eel is known to traverse 

 considerable distances overland. 



.\ still closer approximation to normal atmospheric 

 breathing is exhibited by a ganoid {Lepidosieus), and 

 by certain Brazilian fishes {Sudis gigas, Erylliriiiiis 

 /(Tuia/i/s, and brasilieiisis), which, according to Jobert, 

 speedily suffocate if the air-duct between the air-bladder 

 and the throat is ligatured, becau.se in these fishes gill- 

 breathing does not alone sufiice for the necessary 

 oxygenation of the blood. 



But even in these cases the general course of the 



• In the air bladder of Siluroids and of some other fishes [e.g.. 

 Tench) the rctia mirahilut are absent ; in these cases a peculiar 

 chain of tiny bones (the Weberian ossicles) connects the air- 

 bladder with the inner ear. By means of these ossicles the fish is 

 mate aware of any increased pressure of air in the bladder caused 

 by sudden proximity to the surface of the water. Relief from this 

 pressure is obtained by the emission (doubtless by refle.x action) of 

 bubbles through the airduct and the mouth. The fish is thus 

 able to regulate its position to a plane of least muscular effort, in 

 which it becomes of the same weight, bulk for bulk, as the sur- 

 rounding water. 



circulation has not been finidamentally .altered. To find 

 this state of things we must turn to the dipnoi, the 

 double-bicathcrs, or lung-fishes as they have been aptly 

 termed. In this ancient group of fishes, we ixn.- able 

 to trace tr.ansitional steps in the supplanting nt ^ill- 

 respiration by lung-breathing. 



In Ccralodiis (the least specialised genus), the .-lii- 

 bladder is still imp;iired and dorsal in position, but a 

 slight median longitudinal depression ioresh.-idows the 

 p.airrd condition of the hings of hii^hcr animals. The 

 opening ol the air-duct, .•ilthough lati'ral, opens at the 

 glottis on the right side of the pharynx approximating 

 to the median ventral position of true lungs (i'ig. 3 6'). 

 The latter condition is attained by the African I'ro- 

 toptcnix and the South American Lcpidosircit; and in 

 addition, the lung is paired (Fig. 3, V). The plan of 

 the circulation is modified, for the lung is now suijplied 

 with blood by a true pulmonary artery arising from the 

 point of union of the fourth efferent branchial artery 

 with the aortic root, while in Ctraindus it is still given 

 off from the former alone. 'l"he aiirated bloixl riturns 

 by a pulmonary vein to the heart through the sinus 

 venosus. In Prolnplcrus^ too, the conus arteriosus is 

 completely divided into two halves, so that an arterial 

 and a venous current pass out from the heart side by 

 side, while in Ccraiodtis this diyision is still incom])lete. 

 In all the members of the dipnoi, the lining memljrane 

 of the air-bladder is thrown into ridges and folds just 

 as in the hollow^ sack-like lungs of typical amphibians. 



To this transitional group, therefore, we must turn 

 to find fossil remains intermediate between fishes and 

 amphibians. Their antiquity is great; the few existing 

 species are the widely distributed remnants of a group 

 which flourished in the later Paljeozoic ages. The 

 .\ustralian Barraniund:i {Ccraiodiis) is, indeed, a highly 

 remarkable instance of the persistence of a gencrali.sed 

 type, for the genus existed in the Trias of Europe, and 

 even in the Permian of North .America; the very fact 

 of its possessing low generalised characteristics has 

 enabled it to survive changes fatal to more highly 

 developed and specialised creatures. 



TKe Pleiades. 



For comparison with the plate in the January number, 

 we reproduce this month another photograph of the 

 Pleiades, taken by G. W. Ritchey with the2-feet reflector 

 at the Yerkes Observatory. 



The plate had an exposure of three-and-a-half hours, 

 and shows the principal stars involved in a fine tracery 

 of nebulosity; the star Atlas," with its companion 

 Pleione, which brings up the rear of the Pleiades, is not 

 shown in this reproduction. 



The small photograph, taken by W. Shackleton with 

 a 3i-inch lens, exposure 33 minutes, shows the group as 

 seen with a pair of opera-glasses, or a little more than 

 the naked-eye \iew. 



* The names of the Pleiades stars, with diagram, are given in 



" Knowledge " for January, 1889. 



