;6o 



NATURE 



[Oct. 5, 1882 



numbers near land. There are further numerous animals which 

 are pelagic only in their larval condition, and which, swarming 

 at the surface with the strictly pelagic forms during their early 

 stages, sink to the bottom when mature to lead an entirely 

 different existence. With other pelagic forms, the converse is 

 the case : the pelagic snakes, turtles, and birds come on shore 

 to rear their young, spending most of their adult life on the ocean, 

 and certain whales approach the shore at the breeding season. 

 These two last groups may be termed hemipelagic. 



It is impossible to draw a sharp line between any of these 

 groups ; they run into one another indefinitely. Thus, unlike 

 the abundant flying-fishes (Exoccetus), the flying gurnets (Dac- 

 tylopterus), are never found very far from land, but lead a partly 

 pelagic existence, taking frequent flights from the surface, and 

 partly inhabit the bottom in shallow water, being taken some- 

 times at the bottom with a hook and line. Amongst the Hydro- 

 medusre and Scyphomedusa;, all gradations of pelagic habit 

 occur. Many of both are attached at the sea bottom at certain 

 stages in their life history, or rest on it habitually, some possibly 

 in very deep water ; others, closely allied, exhibit no fixed con- 

 dition, and are entirely pelagic. 



The Sargasso Sea has a peculiar fauna of its own, which 

 cannot be considered as strictly pelagic, composed of animals 

 specially adapted to cling to the gulf-weed and inhabit it, and 

 differing in general aspect from other pelagic forms. Very 

 much has been written on this fauna, which is so special that it 

 may well be left out of consideration here. 



Most characteristic of pelagic animals is the almost universal 

 crystalline transparency of their bodies. So perfect is this 

 transparency that very many of them are rendered almost entirely 

 invisible when floating in the water, whilst some, even when 

 caught and held up to the light in a glass globe, are scarcely to 

 be seen. The skin, nerves, muscles, and other organs, are abso- 

 lutely hyaline and transparent in these forms, but natural selection 

 seems to have been unable to render colourless the liver and 

 digestive tract in many instances. So these parts remain opaque, 

 of a yellow or brown colour, and exactly resemble, when seen 

 in the water, small pieces of floating sea-weed. A familiar 

 example is Salpa, Pelagonemertes is another. 



Certain few pelagic animals are coloured bright blue for pro- 

 tection, so as exactly to resemble the colours of the waves. Such 

 are Minyas cceruleus, Velella, Porpita, Physalia, Glaucus, Ianthina, 

 all of which are forms which float at the very surface, with part 

 of their bodies more or less out of the water. The blue colora- 

 tion seems to be connected with this latter circumstance, as pro- 

 tecting the animals probably from predatory pelagic birds, to 

 which they would be invisible at any distance. Velella does not, 

 however, thereby escape its enemies altogether, for a young turtle 

 which we caught at sea during the Challenger expedition, had 

 its stomach full of Velellas, and we often found them in the 

 stomachs of albatrosses. Ianthina, the well-known bright blue 

 gastropod, constructs a float built in compartments, which is 

 attached to its foot. If this float be detached, the animal sinks 

 and dies. It is said to be devoid of eyes. Glaucus is a nudi- 

 branch mollusc, which has the sides of its body modified into 

 curious fin-like fringed lappets. It floats habitually with its 

 ventral surface upwards, its foot being applied to the surface of 

 the water, just as is that of the common pond snail, Paludina, 

 when the animal is creeping at the surface of the water. In 

 consequence of the position thus assumed by Glaucus, its 

 ventral surface is coloured deep blue, whilst its dorsal or 

 under surface is of a glistening lustrous white. One is so 

 accustomed to animals floating with their back upwards, and 

 being coloured accordingly dark on the back and light under- 

 neath, that the appearance of the animal is most deceptive, and, 

 indeed, entirely misled Dr. Bennett, who, in his account of the 

 habits of the animal, speaks of the blue aspect of its body as its 

 back throughout. The curious fish, the Remora, 1 which adheres 

 to sharks and ships, is similarly dark on the exposed ventral 

 surface, and light on the back, and one can hardly persuade 

 oneself of the fact when one looks at one in the fresh condition. 

 The circumstance proves how completely the arrangement of 

 such colouring is protective in object. 



Glaucus is most persistent in maintaining its position with 

 its back turned downwards. I turned one over several times. 

 It straggled with its fins somewhat like a turtle on its back, and 

 quickly regained its position. Curiously enough, according to 

 Dr. Bennett, it feeds on Velella, which, like itself, is blue. 

 Similarly the blue Ianthina feeds on the blue Velella. 



1 This fact seems not to be recorded by iclhyologists nor figured. When 

 the fish is put in spirits the light tint of the dorsal surface disappears. 



Some few pelagic animals are most brilliantly coloured, and one 

 small Copepod Sapphirrhina has always excited the admiration 

 of naturalists, being unsurpassed by the brilliant metallic lustre 

 of the humming birds, and displaying all the colours of the 

 spectrum with an intensity like the gleam of the diamond. 

 The figure of this animal now on the screen appears brilliant 

 enough, but it gives but a faint idea of the actual brilliancy 

 of the animal. The colouring in this case is of sexual import, 

 being confined to the males. 



A further remarkable fact about pelagic animals is that 

 very many of them have either no eyes or very large eyes, the 

 latter condition being most common. Thus the whole of the 

 Pteropods have either no eyes or mere rudiments of them, and the 

 Siphonophora and Ctenophora have no eyes. 



On the other hand, animals with huge eyes in proportion to 

 their size are common in the pelagic fauna. As an example, I 

 shall throw on the screen a representation of the remarkable 

 pelagic Amphipod Phronima sedentaria, you observe the enor- 

 mous size of the compound eyes, which occupy the entire front 

 of the animal's body. The female Phronima sedentaria has the 

 curious habit of living in a tub-shaped transparent house, open 

 at both ends, whicn it forms by gnawing out the inside of a 

 young Pyrosoma colony, and, with its brood of young clustered 

 round it inside, it sculls its tub with great rapidity through the 

 water. 



Here you see another crustacean, a Copepod of the genus 

 Corycreus. All the species of Coryrceus have a very large pair ot 

 eyes ; but in the present form the eye apparatus is so extraordi- 

 narily enlarged that a large horn-like outgrowth of the body has 

 been formed projecting from under the thorax, in order to accom- 

 modate the nervous structures and get a long enough focus for the 

 lenses. This figure is from an unpublished drawing by my 

 lamented colleague on board the Challenger, Rudolph von 

 Willemoes Suhm, who specially devoted himself to the investi- 

 gation of pelagic animals during the Challenger voyage. He 

 names in MS. this curious form, which is apparently as yet un- 

 described Corycasus Megalops. The animal is of a fine blue 

 tint when living. Most remarkable of all for their eyes are, 

 however, perhaps the pelagic annelids, the Alciopidae. Their 

 eyes are of enormous size and most perfect construction, and far 

 surpass in both respects those of all other annelids. 



In thus being blind or provided with extraordinary organs 

 of vision, the members of the pelagic fauna resemble those of 

 the deep-sea fauna, and there are other points of resemblance 

 between the two assemblages of animals, such as that amongst 

 both a large proportion of phosphorescent animals occur. 

 Prof. Fuchs, 1 in lately-published most valuable papers on the 

 Pelagic Flora and Fauna, and on Deep-Sea Life, has dwelt 

 much on these resemblances, and concludes that they are to be 

 explained by the circumstance that, like the deep-sea fauna, the 

 pelagic fauna is to a very great extent a fauna of the darkness, 

 the deep-sea fauna living where darkness, as far as sunlight is 

 concerned, is perpetual, and the pelagic fauna being nocturnal in 

 its habits. By far the greater part cf the pelagic fauna is thus 

 nocturnal in its appearance at the surface. In the day-time the 

 animals composing it sink to considerable depths, and they rise 

 only at night. Certain pelagic animals, however, seem not to 

 mind the sunlight. Radiolarians may be seen at the surface 

 when it is calm, in the full glare of the sun, and so may Velellas 

 and lanthinas ; indeed these latter and some others cannot leave 

 the surface. Some Ctenophora, especially Eucharis, according 

 to Chun, seem rather to like the sun. Flying-fish, again, are at 

 the surface day and night, and the beautiful pelagic fish called 

 dolphins (Coryphaena) show their wonderful colours to best 

 advantage in the full sunlight, as they swim lazily round a 

 becalmed vessel. Winds and storms drive all the pelagic 

 animals below which are capable of descending, and one may 

 sail over wide tracts of sea during boisterous weather and imagine 

 that the water is almost barren of life, whereas a calm night 

 would have shown the whole surface teeming with animals. 



The important question of the day with regard to pelagic life 

 is, to what depths does it extend ? How far do the animals 

 which come up at night descend, and do any which never come 

 to the surface extend their range below the limit of these again ; 

 and do any inhabit the region leading down to the very deep 

 sea bottom ? 



Prof. Weissmann, 2 from his observations on what may be 

 called the pelagic fauna of Lake Constance, has shown that the 



1 Tb. Fuchs, "Ueber die Pelagische Flora und Fauna, u.f.w." J. C. 

 Fischer, Wien, 1882. 



3 Das Thierleben in Bodensee. Von Aug. Weissmann. Lindau, 1877. 



