218 



KNOWLEDGE 



[No\-EMBER 1, 1895. 



pulling it along the surface. Many fables have collected 

 round the remora, which was known to Aristotle, and in 

 Ovid's time it was believed to have the power of stopping 

 a vessel on its course ; although, how this was effected the 

 poet is careful not to say. The dark coloration of the 

 lower parts is due to the circumstance that, when attached 

 to a foreign body, the fish generally have this surface 

 turned upwards. It has indeed been suggested that the 

 fish habitually swims, even when detached, in the reversed 

 position, but this has not been confirmed. 



By being carried about, the remora, of course, gets a 

 much better chance of obtaining food than would be the 

 case if it had to depend upon its own powers of locomotion ; 

 but according to the observations of F. D. Bennett, the 

 duration of the fish's adherence to foreign bodies is much 

 less than commonly supposed. Thus he writes that "it 

 often merely swims around the body it attends, and only 

 fixes upon it occasionally, and for a very short time. The 

 adhesion of the buckler is chiefly effected by the smooth 

 membrane that margins it. After the death of the fish, 

 and even after the head has been separated from the body, 

 the moist membranous border of this organ adheres to a 

 plain surface with undiminished power. One muscle can 

 be raised and depressed by the fish independently of the 

 others, or all can be moved simultaneously and rapidly. 

 Their uses are, to fix the sucker more firmly, to offer 

 resistance in one determinate direction, and probably to 

 liberate the sucker by relieving the vacuum." 



Not the least curious feature connected with the remora 

 is that it is a member of a family of which the other repre- 

 sentatives have no adhesive disc; and it has a very near ally 

 in a fish from the warmer coasts of the Atlantic and Indian 

 Oceans, known as Elacatc nii/rn. Hence the development 

 of the suctorial disc is a peculiar and specialized feature 

 which has not been inherited from other genera. That the 

 evolution took place at an epoch comparatively remote is, 

 however, proved by the occurrence of a fossil sucking-fish 

 in the lower Eocene slate of Glarus, in Switzerland. 



To find an adhesive organ comparable to that of the 

 sucker-footed bats, we have to go to the molluscs, where 

 the cuttlefish and squids have these organs developed in 

 great numbers on the tentacles, or so-called arms, sur- 

 rounding the head. In most cases these are arranged on 

 the non-pedunculate arms in rows (usually four or two), 

 becoming smaller and more numerous towards the tip of 

 the arms ; and whereas in some forms their base is flush 

 with the surface of the latter, in others they are raised 

 on short stalks. Then again, their 

 margins may be strengthened by a horny 

 rim, either smooth or crenulated; and 

 sometimes their centre is armed with a 

 strong hook, which can be protruded or 

 retracted like the claw of a cat. Struc- 

 turally, according to the description of 

 Owen, these disc3 have a slightly 

 swollen margin, from which a series of 

 muscular folds converge towards the 

 centre, where a circular aperture con- 

 ducts to a cavity of gradually increasing 

 width. Within this chamber is a kind cf button, which 

 can be elevated or depressed in the samo manner as the 

 piston of a syringe, so that when the sucker is applied and 

 the piston withdrawn, a vacuum is immediately created. 



It would be very interesting if we could trace the gradual 

 evolution of these highly specialized organs, but this is 

 unfortunately impossible. They may, however, have 

 arisen after cuttlefish were evolved as a distinct group, 

 since they are unknown in the allied group of cephalopods 

 to which the nautilus belongs ; but they arc now constant 



Fia. 3— Sucker 

 Cuttle-Fish. 



of 



throughout the group, and thus differ markedly from all 

 the sucking organs hitherto mentioned. In the gigantic 

 cuttles occasionally met with in the ocean, they attain an 

 enormous size, beintj, it is said, in some cases, of the size 

 of dinner-plates. While these suckers are employed to 

 anchor their owners to submarine rocks, they are likewise 

 used for the purpose of seizing prey and bringing it within 

 reach of the jaws, and in this latter respect differ from 

 all those hitherto noticed. 



Among insects, as already mentioned, many exquisite 

 examples of adhesive organs are to be met with, but the 

 only ones we have space to refer to here are those on the 

 fore-legs of the males of many of the water-beetles of the 

 family Diiticida. In the common English Ihjticus the first 

 joints of the tarsus are widely expanded so as to form a 

 nearly circular plate ; this being provided with a number 

 of suckers, one of which forms a very large disc, furnished 

 with strong radiating fibre, while a second and smaller one 

 is of the same type. Besides these are a number of small 

 tubular and somewhat club-shaped bodies, each furnished 

 with a very delicate sucker at its extremity. We have 

 been unable to come across any account of the use to 

 which these very beautiful examples of adhesive organs 

 are appHed, but their occurrence in the male sex alone 

 may suggest one to our readers. 



In all the foregoing instances the adhesive organs are 

 formed either on the body or limbs, and, with the exception 

 of the cuttles, are used solely as a means of attachment. 

 The lampreys (Petromydda:) 

 and leeches {Hirudin idee), 

 which will be our last ex- 

 amples of these curious struc- 

 tures, afi'ord, on the other 

 hand, instances where the ,..j,, 

 mouth is modified into an 

 adhesive organ. In the lam- 

 preys, which although formerly ''' 

 included among fishes, are 

 now generally regarded as 

 constituting a class by them- yiq. 4. 

 selves, the mouth is surrounded 

 by a continuous circular or 

 suboval sucking lip, while internally it is armed with 

 a variable number of sharp teeth resting on a soft 

 cushion. Lampreys feed on the flesh of fishes, to whose 

 bodies they attach themselves by means of the suctorial 

 mouth, and when thus firmly fixed, rasp away the flesh 

 with their teeth. Frequently they cling for a long time to 

 the surface of their victims, and are thus carried long 

 distances. The occurrence of fossil lampreys in the older 

 Palaeozoic rocks of Scotland proves that this type of 

 sucking mouth is at least a very ancient, and probably a 

 primitive one. 



Most likely the same is the case with the leeches, which, 

 as our readers are doubtless aware, belong to the great 

 group of annelids, or worms. In these, the anterior end 

 of the body terminates in a large circular sucker, within 

 which, or the pharynx, is the mouth. Very generally, as 

 in the common medicinal leech, the mouth is armed with 

 sharp teeth, and thus presents a curious structural 

 similarity to that of the lampreys, although there is, of 

 course, no genetic connection between the two groups. 

 Many leeches also have a sucker at the opposite extremity 

 of the body, by which they are in the habit of attaching 

 themselves to the leaves of trees, from which basis they 

 extend their bodies in the hope of catching hold of a 

 passing animal. In their habits leeches also present a 

 curious similarity to lampreys, except that they only suck 

 the blood of their victims, instead of feeding on the flesh. 



kinft-moutU of 



-S 

 Lamprey. 



