Oct. 5, 1882] 



NATURE 



56i 



minute Crustacea of which it is composed, slowly rise and sink just 

 like the pelagic inhabitants of the sea. They never sink to a greater 

 depth than twenty-five fathoms, but from this depth they rise 

 g-adually to the surface in the evening, following the limit of 

 darkness, as the sun sets, and they descend in the same manner 

 in the morning as the sun rises. Prof. Forel has observed the same 

 facts in the Lake of Geneva. Now a depth of about twenty-five 

 fathoms has been shown by Prof. Forel to be the limit at which 

 sensitised paper ceases to be acted upon by direct sunlight in 

 the waters of the Lake of Geneva. Below this depth no sun- 

 light penetrates. Prof. Weissmann, after trying all other 

 apparent explanations, concludes that the reason why the pelagic 

 cru>taeea oscillate perpetually in this curious manner in depth is 

 in order to economise the light and be able to feed during the 

 twenty-four hours over their whole possible range of feeding- 

 ground. Were they to remain at the surface during the daylight 

 they could not see at all to feed in the depths in the weak light 

 at night. This most ingenious explanation will no doubt apply 

 equally well to all the marine pelagic animals with well-deve- 

 loped eyes, and which feed on the nearly stationary vegetable 

 matter and dc'bris held in suspension by the surface slrata of the 

 waters. Whilst the numerous blind forms which execute similar 

 diurnal oscillations, such as the Ctenophora, 1 Echinoderm 

 larva;, 2 Pteropods, and others, must follow the Crustacea and 

 other small fry to feed upon them. Indeed the whole pelagic 

 fauna is so closely inter-dependent, that it must needs move 

 together. It is very probable that some forms come to the 

 surface only at night, because thereby, like so many other 

 nocturnal animals, they escape many possible enemies by always 

 keeping in the dark. 



Dr. Chun has observed that the Ctenophora of the Gulf 

 of Naples, after being abundant in spring, become ex- 

 tremely scarce and almost di.-appear during the three summer 

 m mlhs, and re-appear suddenly again in great numbers in the 

 autumn. He believes, from hiving caught them in the summer 

 at considerable depths, that these Ctenophora descend annually 

 at the end of spring in order to feed on the minute Crustacea 

 which then remain in deep water (very possibly because the 

 more powerful light allows them then to feed at the lower level), 

 and that, having become fully fed up, and the young having in 

 the depths passed through their metamorphoses and reached the 

 adulti condition, they rise together to the surface, and appear 

 in a swarm as if by magic. One of the Ctenophora with this 

 habit is the beautiful venus girdle ( Cestus veneris). Scypho Medusa; 

 (Cassiobeia Borbonica) and other pelagic animals, appear to per- 

 form the same periodical migration in depth. Doubtless similar 

 annual migrations in depth occur amongst pelagic animals in 

 various parts of the world, and this may account for the extra- 

 ordinary scarcity of some few. 



It appears probable, therefore, that pelagic animals per- 

 form oscillations in depth from three different causes. They 

 perform, firstly, diurnal oscillations in accordance with the 

 changes in light and darkness ; these, secondly, are liable to 

 constant interruptions from the occurrence of boisterous weather ; 

 and thirdly, they may alter their depth periodically, according 

 to the season of the year. 



The great inland fresh-water lakes have each a regular 

 coast or littoral fauna, a deep-sea fauna, and a pelagic 

 fauna, just like the oceans. The pelagic animals of the 

 lakes resemble those of the sea in many interesting particu- 

 lirs. They are, like them, hyaline and transparent, of most 

 curious forms, modified for a constantly swimming existence, and 

 sometimes possess immensely developed eyes. 1 shall throw on 

 the screen figures of two crustaceans from the pelagic fauna of 

 the Lake of Geneva, from Prof. Weissmann's figures. Both are 

 Cladocera or water-fleas, of the one-eyed family, Polyphemidae. 

 The first, Bythotrepbes, is of most extraordinary shape, having 

 an enormously long tail spine to balance its top-heavy body ; it 

 is transparent like glass, but in late autumn becomes covered 

 with beautiful ultra-marine spots. It has a single enormous 

 compound eye in front, and in the brood pouch, under the 

 rounded carapace on the back is born a single egg. The 

 second, Leptodora hyalina, is also of most extraordinary form ; 

 it is absolutely transparent, like Bythotrephes, and almost 

 invisible in a glass of water. It has an enormous pair of 

 feathered rowing antenna; to sustain it in the water. This 

 curious animal, as well as a species of Bythotrephes, has lately 



1 Dr. Carl Chun, Fauna and Flora des Golfes von NeapeL Ctenophora, 



s. 219- 



2 A. Agassiz, North American Star Fishe: 

 H rvard, 1877, p. 28. 



Mus. Comp. Zool. 



been discovered by Mr. Conrad Beck in Grasmere Lake, in 

 Westmoreland, together with other Cladocera, so that our own 

 lakes have their pelagic fauna. Leptodora hyalina had previously 

 been found by Mr. Bolton in the Olton reservoir near 

 Birmingham. 1 



But the most important question, as I said before, is to what 

 depth do the pelagic animals of the ocean descend ? This has 

 remained an unsolved problem ever since it first exercised the 

 mind of the great Johannes Muller, though in his time the question 

 was a different one, being directly connected with that of whether 

 there was any life at the deep-sea bottom or not. An open net 

 sent down to any depth, as it comes up may catch animals at 

 any intermediate depth. Hence it is impossible to assign to any 

 particular depth with any certainty any animals found in a tow- 

 net when raised to the surface. What is required is experiments 

 made with a net so constructed as to be sent down closed to a 

 certain depth, then opened, then towed for some distance, and 

 then raised again to the surface. Such a net has been de- 

 vised by Capt. Sigsbee, of the U.S. Navy, the inventor of nearly 

 all the best deep-sea apparatus now in vogue, and has been used 

 by Mr. Alexander Agassiz, who found that the pelagic animals on 

 a calm day extended pretty uniformly downwards from the surface 

 to a depth of 50 fathoms, but that at depths of more than 100 

 fathoms nothing was to be caught at all. Unfortunately very 

 few experiments have as yet been made by Mr. Agassiz with 

 the instrument, and therefore no final conclusions can be draw 11 

 from them. We look forward with the greatest interest to 

 further prosecution of similar researches. 



On the other hand there is evidence pointing to a further ex- 

 tension in deptli of deep-sea forms. On board the Challenger my 

 colleague, Mr. John Murray, throughout most of the voyage, 

 made very numerous experiments with the tow-net at great depths, 

 and so constantly obtained very different results by these means 

 to those which were shown by nets simultaneously worked at 

 intermediate depths that he is firmly persuaded that the Pelagic 

 Life extends to very great depths, indeed certain animals which 

 he caught such as the Phoeodaria which have been described by 

 Prof. Haeckel, were obtained only from nets which had been 

 down to very great depths. It is indeed possible that there is a 

 direct connection between the deep sea fauna and th it of the surface 

 and that the young of certain deep sea fish pass their early 

 existence at the surface amongst the Pelagic throng. It is known 

 with certainty that the young of many fish living in tolerably 

 deep water, such as the cod, inhabit the surface water in their 

 early stages, and it is possible that the eggs of fishes living at 

 great depths may similarly rise tc the surface for development. 

 Prof. Lutken 3 has described a small fish which was obtained from 

 the stomach of an albacore which appears without doubt to be 

 the young of a deep sea Lophoid, probably Himanlolophus 

 rheinhardlii, and the young of other deep sea fish have been 

 found under similar circumstances. 



Mr. Agassiz, whose authority on the matter is of the greatest 

 weight, is nevertheless convinced 3 "that the surface fauna of the 

 sea is really limited to a comparatively narrow belt in depth, and 

 that there is no intermediate belt so to speak of animal life 

 between those animals living on the bottom or close to it and the 

 surface Pelagic fauna." If this be the case then the limit in 

 depth from the surface must be ultimately due to the limit in 

 the penetration of sunlight, and consequent growth of vegetable 

 organisms. Over this belt the ultimate source of food of the 

 Pelagic and deep sea animals is concentrated ; below it a con- 

 stant rain of organic debris is always falling slowly, 4 through an 

 immense interval of absolute darkness to the deep sea bottom, 

 but this rain thus spread out is sparse, and being so, it could 

 scarcely be obtained by animals devoid of or unable to employ 

 vision in sufficient quantity to support life. 



If the intermediate zone is permanently inhabited at all, such 

 habitation probably depends on the p losphorescence of the 

 animals dwelling in it whereby they are able to use eyes and 

 pick up the scanty food. It is quite possible that some of the 

 fishes endowed with phosphorescent organs such as the Scope'., 

 which, as Dr. Giinther reports were brought up in the Challenger 

 nets "from any depth to 2,500 fathoms," and some of which 

 occur on the surface, may roam through the intermediate zone 

 finding food by means of their own light, and that may be the 



' E. Ray Lankester, Aim. ami Mug. rVal Hist., January 1882, p. 53. 

 = Vidensk. Selsk. Skr.. 5" Ra-kke 11", Bd. v. 



3 Bull. Mus. Comp. Zool. Harvard, Vol. VI., No. 3, p. 153 



4 By experiment I fjund that a dead Salpa would take about four days to 

 reach the bottom in a depth of 2,000 fathoms. (" Notes by a Naturalist en 

 the Challenger." Mac.nillan, 1879, p. 582.) 



