SOME CONNECTING LINKS.

Otago Witness, Issue 1932, 30 November 1888, Page 35

SOME CONNECTING LINKS.

; v As •we might reasonably enough, expect, undoubted transitional forms '.are more s numerous in the lower orders of the animal .kingdom, than • among the higher— until, indeed, quite towards the lower end of the, chain the gradations are so gentle that often great difficulty is experienced in assigning a given-form its^pfdper .plAGfiJJOhg . classified list.' I will not attempt, therefore, to. enter the great subdivision or the inyerte-; brates, bat,will rest content with briefly considering a few of ' ' ' " The Hfost Striking 1 F|ormB Among the fi ' Vertebrates, . ' : . leaving tb,ose of my readers who ,wish l to i pursue the subject further to read for them-, selves some good work on zoology. The last of the'' connecting links; ,then/I will have to deal with ar£ the intermediate forms between true ushes-^the lowest f .vertebrate.Sr-and reptiles. -It will not be necessary to describe the 1 general characteristics 6f, fishes further thanto state that ■fhe'y breathe' by gills, and that the heart conaistsof two chambers only, one auricle' and one ventricle. The auricle receives the blood gathered up from thegeneral system and from the gills, while the ventricle despatches mixed blood to the system and to the gills. The oxidation of the blood is thus accomplished very slowly, and in consequence the temperature of that fluid remains low. At the very top of the list of fishes there is an Undoubted Connecting JLink in the lepidosiren, a mud fish) found in the swamps of the Amazon and the Gambia. In periods ,of drought this peculiar fish burrows in the mud, where it forms a cavity in which it remains dormant till returning rains bring water. It is manifest ; that nc true fish breathing water by gills cpuld exist for an indefinite time without water, so that we must inquire a little more closely into the structure of the lepidpsiren to find out wherein it differs from the true fishes. It resembles fishes in the shape of its body, its true fish scales, and the undoubted- fishlike fin which clothes its tail. It possesses gills of the ordinary fish character, but the heart consists of three chambers, two auricles and a ventricle. The swim bladder is so modified as to form true lungs. Thus it is that the creature can exist for a time at' least as an air-breathing animal. The right auricle receives the venous blood from the body, while the let -> one receives the aerated blood from the lungs. The common ventricle despatches mixed venous and arterial blood to' every part of the body.* While living under normal conditions, how-, ever, the circulation will be shut off from the lungs and diverted to the gills. According to circumstances the lepidosiien exist 3 as ai true fish breathing air dissolved in water by means of gills, and using -two chambers of, its heart only"; ' or under altered ciroum- 1 stances breathes air direct by means of, lungs, employing' the 'three chambers of its 1 heait to keep going the necessarily more; complicated' circulation. In the earlier stages of life this creature has a fringe of external'gills like th. young of some of the; true amphibians. Altogether the lepidosiren strikingly shows its relation to fishes on the; one hand and amphibians on the other. ; • Passing over" the lowest true amphibians j certain snake-shaped animals 'found in Sottthj America, Java, and Oeylon, we-will next hold in review. . t -•; .k . ■ r' 1 -,„" ' ' • The Proteus, i found in certain caves in Illyria and Dal-| matia. This, little creature,, which only; attains the. length of one ;foot, r possesses aj somewhat fish-like body of a pale flesh ,pt, nearly white colour,.with three pairs of bright! scarlet gills on each side. of the neck externally/ It has -two pairs 6f 'very "weak limbs, the fore limbs having three toes an^ and the hind ones Two. Living in dark; leaves as it does, the power of vision must be| quite useless- to it, so that we need not. marvelj ah its eyes being rudimentary and covered, over by the skin. The proteus is a true amphibian and can live either in the water^ breathing by means of its external gills, or! breathe air direct by its lungs, while the cir-j culation of its blood is kept up by its three-, chambered heart. One peculiarity of the proteus is that its oval-shaped red blood cor- ( puscles are larger than those of any other vertebrate. i

The Mexican Axolotl

grows to about the length of a foot or 14 inches. It has a fish-shaped body, but thej head departs considerably from the fish typej It has four limbs, much fuller developed than those of theproteus, the fore pair" havitiglour toes each and the hind pair five. The ex^ ternal gills form a fringe round the neck-, andj are retained permanently through life, or at any rate the animals breed freely in this conr dition. Experiments have shown that under confinement an ally of the axolotl, the siredon lichenoides of the Western States of America, loses its gills and breathes by its lungs only. Its dorsal and candal fins also drop off, while it becomes less aquatic in habits. These changes go towards proving^ what Cuvier long' ago suspected, that' the species of siredon are all probably lar« val salamanders. The water salamanders, or newts, have a fish-like tail. 'The young are tadpole-like, with eternal gills, which are lost in the adult, and replaced by longs, as in the common frog. While the newts are all oviparous, their immediate descendants i 4 the rising scale, the land.salair^ndcrßjare ovoviviparous, or produce their young-alive^ in which case the external .gills are sbmetimes lost before birth. " The land : salarh,anj ders are extremely lizard-like 5 in the adult 'stage, but the possession of gills in the larval stage distinguishes them from lizards, which; never breathe by gills at any stage of their existence. -

The frogs again complete the chain, linkedby gradations which may be easily traced from fishes through the order of amphibians to the true reptiles. The development ofj the frog exhibits all the phases of life common to the three great divisions. Firstj the egg develops into a distinctly 'fish-like creature, possessing internal gills like a fish, which it much resembles In shape, while it has at the same time external gills in common with the larvae of all amphibians. The lattei are the first to drop off, and at a later stage o£

growEßthe internal gills arereplaced by lungs. The frog exhibits a good illustration of the general zoologioal law that the phases of development of the lower members of any great , group of animals are summed up, so to speak, in the development of the higher members of the group. Characters which are permanent" in the lower amphibians are merely transient and embryonic in the frog. The pursuit of this subject is very .interesting, but' I will not follow it farther than to summarise what we may, learn of Nature's laws, and the plan of creation from the brief examination of the few connecting links we have merely glancedat, -. - n *-* * " ,-