Part 7 (1/2)
It has already beenon the sea-bottoreatly in character froo farther out from land, however, into the deeper water, the character of the fauna gradually changes One by one the species found near the shore become rare and disappear, and their places are taken by others characteristic of the interive way to others, till in the abysses of the great oceans we find an assee anireat depths, and differing widely in many respects from the more familiar inhabitants of the coastal waters In this ”fauna of the deep sea,” which extends to the greatest depths reached by the dredge or trawl, the Crustacea occupy a pro to discuss some of these peculiar forms, however, it is necessary to attempt to form some idea of the conditions under which they live
In the first place, the character of the sea-bottoreatly as we pass away from the coast Near the shore it is extre in one place of rocks swept bare by the tides or overgroith jungles of sea-weed, in another of banks of gravel or shi+ngle, of sand or of mud, but in all cases derived from the ”waste” of the land, as it is eaten away by the waves or washed down by the rivers
As the distance from land increases, the deposits become finer and finer, till they shade off into a soft oozy ht down by the rivers In the neighbourhood of large rivers this mud may sometimes extend for hundreds of miles from the land, but there is a limit to the distance to which even the finest particles can drift before they settle to the bottom, and beyond this limit the floor of the ocean is covered by sediin, not to the land, but to the ocean itself The surface waters of the ocean everywhere tee animals and plants, and, as these die, their remains sink to the botto the anisms--in the warmer seas, at any rate--are certainto the lowest class of the anidom, and have shells composed, in most cases, of carbonate of lime Over vast areas the bottorey ooze, made up almost entirely of the dead shells of Foraminifera rained down from above Since the commonest species of Foraenus _Globigerina_, the deposit is known as ”Globigerina ooze”
In certain regions of the ocean the shells of other floating organis the ocean floor, and in the deepest abysses--so deep that the shells of surface animals are dissolved before they can sink to the bottom--there is found a deposit known as the ”red clay,” which appears to be derived largely from the impalpable volcanic and cosmic dust that floats in the atmosphere It is not necessary for our present purpose to enterquestions connected with these deep-sea deposits, but it is i, the floor of the deep sea is everywhere soft ooze, without rocks or stones, except for an occasional water-logged lu This fact is probably of great importance in the life of deep-sea animals
One of the most peculiar and characteristic of the physical conditions in the deep sea is the enormous pressure under which life has to be carried on At the surface of the sea the pressure of the at, 14-1/2 pounds per square inch At a depth of only 33 feet of water this pressure is doubled, and at greater depths the pressure increases in proportion, till at 2,000 fathoms it is more than 2-1/2 tons on the square inch As a matter of fact, however, the animals at the bottom of the sea are probably but little affected by this enore the sudden release of pressure causes the fluids of the body to expand and destroys the tissues, so that the ani when they reach the surface
More important than the pressure in its influence on life is the darkness of the depths The light of the sun only penetrates the water of the sea to a coh light to produce any effect on a photographic plate Even at a considerably less depth the absence of light puts an end to all plant-life, except for the ubiquitous bacteria, and it follows that all the animals of the deep sea ultimately depend for their food-supply on the rain of dead bodies of surface ani on the sea-bottom
The temperature at the bottom of the deep sea is always very low Dr
Alcock states that ”in the open part of the Bay of Bengal, where the mean surface temperature is about 80 F, the temperature at a depth of 100 fathoms is only about 60 F, at a depth of 300 fathoms not quite 50 F; while at a depth of 2,000 fatho-point”
Finally, it is important to notice the _uniformity_ of the conditions at the bottoht and day and the progress of the seasons unfelt in the abysses, but the conditions are practically the same over vast areas in all the oceans
In the case of deep-sea Crustacea, we are frequently confronted with a difficulty which does not occur in the case of soroups of animals--Corals or Echinoder whether the animals really lived on or near the bottom, or were captured by the open mouth of the trawl on its way to the surface When the ani--as, for instance, most of the Crabs--it may be assumed that they did actually live on the bottom; but, with the prawn-like forms, the possibility that they may really be inhabitants of the intermediate depths must always be taken into consideration
[Illustration: FIG 42--A DEEP-SEA LOBSTER (_Nephropsis stewartii_), FROM THE BAY OF BENGAL REDUCED (After Alcock and Anderson)]
[Illustration: FIG 43--_Munidopsis regia_, A DEEP-SEA GALATHEID FROM THE BAY OF BENGAL REDUCED (After Alcock and Anderson)]
In animals that live in perpetual darkness we should expect to find, in accordance with the principle of adaptation which runs through the whole of organic nature, that the eyes are wanting or ireat many deep-sea anienus _Nephropsis_ (Fig 42), which are very closely allied to the Norway Lobster (_Nephrops_) of shalloater, have very short and slender eye-stalks hidden under the rostru at the tip only the merest traces of as once an eye In the lobster-like Eryonidea (see Fig 46, p 133), the reduced eye-stalks are fire of the carapace Some of the deep-sea Crabs and Prawns seeeneration has not quite gone so far, and the eyes are present, although much reduced and modified Thus, the very nuenus _Munidopsis_ (Fig 43) and its allies have, as Alcock says, ”pallid, h they ht and darkness, can never fore” It is probable, indeed, that these pale-coloured eyes are specially adapted for vision in a diht, for it has been shown that in certain deep-sea Euphausiacea the pigment-sheaths between the separate elereatly reduced, and are fixed in the position temporarily assumed by those in the eyes of normal Crustacea when kept in the dark Be this as it may, there are many deep-sea Crustacea which have well-developed and darkly-pig forrate into the upper strata of water to which soht penetrate; but there are so species that have well-developed eyes, although they live at great depths This would seeht of day, they are not conderope in perpetual darkness Many deep-sea animals are known to be phosphorescent, and it seeht eloorms and fireflies of the abyss Thus, Alcock points out that the deep-sea Herurus pilosimanus_ (Plate XVI), which lives in partnershi+p with a colony of Sea-aneh it descends to depths of at least 2,000 fathoests that the Crab ht emitted by the zoophytes
[Illustration: _PLATE XVI_
DEEP-SEA HERMIT-CRAB, _Parapagurus pilosimanus_, SHELTERED BY A COLONY OF _Epizoanthus_ FROM DEEP WATER OFF THE WEST OF IRELAND (SLIGHTLY REDUCED)]
Some of the Crustacea, however, are themselves luminous Thus, Alcock records how specimens of a deep-sea Prawn, _Heterocarpus alphonsi_, ”poured out, apparently frolands' at the base of the antennae, copious clouds of a ghostly blue light of sufficient intensity to illuminate a bucket of sea-water so that all its contents were visible in the clearest detail” Certain other Prawns are known to possess special light-producing organs on various parts of the body and lians have been roup (see Fig 24, p 56) are by noat the surface of the sea, the structure of their luans, or ”photophores,” may appropriately be described here They are situated on the under-surface of the abdos, and on the upper surface of the eye-stalks
Each consists of a globular capsule covered by a layer of pigment, except on the outer side, where there is a transparent biconvex lens In the centre of the capsule is a peculiar ”striated body” which seems to be the actual seat of luminescence, and behind it is a concave reflector co a silvery lustre Before their luans were described as ”accessory eyes,” but there can be little doubt that they serve rather as searchlights, although, from the positions that some of them occupy on the body, it is not easy to see how they can illue of the eyes That the function of phosphorescent organs is not always that of giving light for their possessor to see by is shown by the fact that many luminous animals are blind It is important to notice, however, that these blind animals never have complex ”photophores” like those just described, but only exhibit a diffuse lu Crustacea, the blind Eryonidea (see Fig 46, p 133) may be mentioned, one species of which was observed by Alcock to be ”lus, where there is a triangular glandular patch” In a recent discussion of the whole question of phosphorescence in anisms, Dr Doflein concludes that the part it plays in the life of the animal probably differs in the different cases In some it may serve to attract prey, as moths are attracted to a candle; in others it ether in a swareht-patterns that serve as ”recognition marks” like the colour-patterns of aniht of day The clouds of luminous secretion thrown out by _Heterocarpus_ and other Prawns, and by certain Mysidacea and Ostracods, may serve to baffle pursuers, like the cloud of ink thrown out by a Cuttlefish, and in soans e of vision That this does not exhaust the possibilities of speculation on the subject, however, is shown by the case of certain deep-sea Prahich have been recently discovered to possess photophores placed so as to illuill cavities What function they can discharge in this position seems beyond conjecture
The colours of deep-sea Crustacea are very curious Few of them have the blanched appearance common, for instance, in animals that live in the darkness of caves; on the contrary, their colours are often very vivid, but they are nearly always unifore proportion of cases are in soe This red colour seems to be associated, in some way that we do not understand, with the darkness of their habitat The general absence ofDr Alcock remarks that in deep-sea Crustacea we never see ”those freaks of colour, or those labyrinthine s, that excite our curiosity when handling the Crabs and Shrimps of the reefs” Possibly the explanation of this may be that in these dwellers in darkness colour is ical processes directed to other ends, not a character of protective or warning value, as in aniht of day It is a curious fact, whichon this problem, that in many cases, while the adults are coloured in soht blue or green
Some of the peculiarities of structure observed in deep-sea Crustacea see about with security on the soft ooze of the sea-floor Aed species, not only aroups (Droured on Plate XIX, and Oxystomata), the members of which assued with long stiff hairs, whichin the ooze, and the spines on the body and legs ofthe deep-sea Prawns, the species of the fa and slender legs, which weover the soft ooze
[Illustration: _PLATE XVII_
A DEEP-SEA PRAWN, _Nematocarcinus undulatipes_ (SLIGHTLY REDUCED)
(_Fro the food of deep-sea animals In the absence of plant-life they must of necessity be all carnivorous, and all ulti from above Some species have been found to have the food-canal filled with Globigerina ooze, which they no doubt s, as earth-worms do the soil in which they burrow, for the purpose of extracting the nutriment that it contains In one species of deep-sea cumacea (_Platycuma holti_), which appears to feed in this manner, the food-canal is coiled, a condition very rare in Crustacea; in all probability this is due to the necessity for an increase of the absorptive surface, since it is co and consequent coiling of the gut, or by infolding of its walls, in anie quantities of relatively innutritious food material Many species, however, no doubt haveThe lobster-like _Thauer_ expedition in the West Indies at a depth of 450 fathoot from deep water off the japanese coast, has one of the chelae enorers set with spines like the teeth of a rake It has been suggested that this re the ooze for small animals on which the _Thauested for the long and spiny first pair of walking legs in the Spider Crab _Platy 45)
[Illustration: FIG 44--_Thaumastocheles zaleucus_ REDUCED (After Spence Bate)]
[Illustration: FIG 45--A DEEP-SEA CRAB (_Platymaia wyville-thomsoni_)