Part 8 (1/2)
[Illustration: FIG 48--_Conchoecia curta_, AN OSTRACOD OF THE PLANKTON
40 (Partly after G W Muller)]
Many of the ic ani continuously afloat with a minimum of exertion The Crustacea of the plankton never carry the heavy ar species Thus, the thick-shelled Ostracoda of the bottom are represented in the plankton chiefly by the fa 48), in which the shell is thin, uncalcified, and almost membranous Many species, particularly of the Copepoda, are seen, under the h the tissues of the body, and these no doubt serve as floats, increasing the buoyancy of the animal The sae spaces, filled with fluid, within the body This is characteristic of pelagic animals, and is well seen in many of the Crustacea in which the viscera and muscles occupy a relatively s spaces being filled with colourless transparent fluid Many of the Hyperid Aigantic _Cystisoma_, which is mesoplanktonic in deep water; and it reaches its extre 49), in which the anterior part of the body is, as it were, blown out into a balloon, giving the animal the aspect of a small jellyfish rather than an Amphipod
[Illustration: FIG 49--_Mimonectes loveni_ A FEMALE SPECIMEN SEEN FROM THE SIDE AND FROM BELOW, SHOWING THE DISTENDED-BALLOON-LIKE FORM OF THE ANTERIOR PART OF THE BODY 3 (After Bovallius)]
If, as seems probable, the body-fluid of these aniravity than the sea-water, it will act like the oil-globules of the Copepoda in keeping the aniravity be the same, however, the distension of the body with fluid acts in another way, by increasing the surface exposed to friction with the surrounding water, and so retarding sinking The principle involved is illustrated by the fact that a soap-bubble sinks h the air than the drop of water into which it collapses The same result is produced if the surface is increased by outstanding spines or hairs, just as, for instance, a downy feather sinks slowly through the air, but drops rapidly if it is rolled into a ball between the fingers This is, no doubt, one function of the spines hich plankton Crustacea, and particularly larvae, are frequently provided, though they ainst ene the various larvae, but it ly developed in larvae which live in the open ocean; for example, the es of _Sergestes_ (Fig 50), which, like the adults, belong to the oceanic plankton The nauplius larvae of Cirripedes are all erated develop 51), of which the adults are attached to floating drift-wood or the like, and belong to the oceanic fauna, although hardly to be classed with the plankton
[Illustration: FIG 50--THE ZOeA LARVA OF A SPECIES OF _Sergestes_, TAKEN BY THE ”CHALLENGER” EXPEDITION 25 (After Spence Bate)]
[Illustration: FIG 51--THE NAUPLIUS LARVA OF A SPECIES OF BARNACLE OF THE FAMILY LEPADIDae, SHOWING GREATLY-DEVELOPED SPINES FROM A SPECIMEN TAKEN IN THE ATLANTIC OCEAN, NEAR MADEIRA 11 (After Chun)]
The large feathered bristles that decorate the limbs or tail of many plankton Copepoda have no doubt the saenus _Calocalanus_ (Fig 52), for exae and brilliantly coloured feathery plumes, and in one species, _C plumulosus_, one of these setae is of relatively enor as the body of the animal itself
[Illustration: FIG 52--_Calocalanus pavo_, ONE OF THE FREE-SWIMMING COPEPODA OF THE PLANKTON ENLARGED (Froy,” after Giesbrecht)]
Aular of plankton Crustacea are the _Phylloso 28, p 72) of the Spiny Lobsters and their allies (Scyllaridea), which have been already described These larvae are sometimes found far out at sea, and it seeed, and that they reat distances by ocean currents At all events, they are well adapted for pelagic life, since the broad flat body, hardly thicker than a sheet of paper, can be sustained in the water like a ”hydroplane” by cos
The watery character of the body, together with the thinness of the exoskeleton, helps to explain the glassy transparency which is a feature of arded as a protective adaptation rendering the animals inconspicuous in the water, and it has indeed that effect to human eyes, but it is very doubtful whether the animals deriveand other pelagic fishes--that prey upon plankton Crustacea appear to s them in bulk, without much selection; and the Greenland Whale, as it swiuided by the greater or less visibility of the Copepods that it sifts out on its baleen plates Further, this glass-like transparency is by no htly coloured In some, as in the beautiful blue _Anoment in the fluids and tissues of the body; in others the feathery hairs on the body and limbs show brilliant metallic colours, produced, like the colours of a peacock's feather, not by pight in the texture of the organ The most beautiful of all Copepoda is _Sapphirina_, in which the surface of the body absolutely sparkles with iridescent colours
The striking phenomenon known as the ”phosphorescence of the sea” is faer, and is seen froht the crest of every wave often seelow, the wake of the vessel is a trail of light, and an oar dipped in the water seems on fire This luely to the lowly Protozoa and the jellyfishes, but in part also to certain Crustacea A nuic Copepoda have been shown by Giesbrecht to secrete, frolands on the surface of the body, a substance which beco in contact with the water Even speciht on being wetted Soic Ostracods of the family Halocypridae have been observed to ehbourhood of the mouth A similar habit has been seen, as already mentioned, in certain deep-sea Prawns and Mysidacea, whichto the deeper part of the ht-producing organs of the Euphausiacea have already been described in dealing with deep-sea Crustacea A great roup, however, are members of the epiplankton, and in these the phosphorescent apparatus is quite as fully developed as in species coica_ (Fig 24, p 56), which is one of the largest of the Euphausiacea, is coreat depths in many places in British seas If a jar of sea-water in which speciht into a dark roolass will cause the photophores to flash out like a row of tiny la for a few seconds the light dies out, to appear again if the tapping be repeated
There are certain peculiarities in the structure of the eyes in soht is of special ih we can hardly do nificance Most Copepoda have only a single eye in the le eye of the nauplius larva, and of far simpler structure than the paired compound eyes of most other Crustacea In many plankton species, however, this sied and complicated in various ways The three parts of which it is normally made up may become separated from each other, and are so to concentrate the light are often developed by thickening of the overlying cuticle The most elaborately constructed eyes are found in the fa 53) a pair of eyes of relatively enore biconvex lens set at the end of a conical tube which extends backwards to a slass and eyepiece), behind which, again, are the sensory cells, corresponding to the retina, enclosed in a tube of dark pigth of the body These eyes, although paired, do not correspond to the paired compound eyes of other Crustacea, but have arisen by the separation and enlargement of two of the three divisions of the typical median Copepod eye
[Illustration: FIG 53--_Copilia quadrata_ (FEMALE), A COPEPOD OF THE FAMILY CORYCaeIDae, SHOWING THE PAIR OF LARGE ”TELESCOPIC” EYES x 20
(After Giesbrecht)]
A peculiarity of the paired compound eyes found in plankton Crustacea of several different orders consists in the division of each eye into two parts, which differ in structure Inthe deeper strata (mesoplankton), this division of the eyes is wellthe separate elethened and with reduced pigment, while the lateral part is of more normal structure It seems probable, from the researches of Professor Chun, that the fronto-dorsal division is adapted for the perception of very faint light, while the lateral division will give a htly illuminated objects
[Illustration: FIG 54--_Phronima colletti_, MALE FROM A SPECIMEN TAKEN IN DEEP WATER NEAR THE CANARY ISLANDS 12 (After Chun)]
In the pelagic A the suborder Hyperiidea, the eyes are of very large size, generally occupying al the animals a very characteristic appearance, in contrast to the s Ga 54) the eyes are each divided into two parts, differing in structure in the way just described
There are a few Crustacea living habitually on the high seas which cannot be reckoned as belonging either to the true plankton or to the necton, since they depend on outside help for keeping the these are the Barnacles which cluster on logs of drift-wood, and are a” of shi+ps' hulls on long voyages The stalked Barnacles of the genus _Lepas_ are especially common in such situations, and the characters of their larvae have been already alluded to Certain species of sessile Barnacles are constantly found attached to large marine animals For example, _Chelonobia_ adheres to the shell of Turtles, while _Coronula_ and soenera are found on Whales
[Illustration: _PLATE XIX_
_Latreillia elegans_, ONE OF THE DROMIACEA WHICH RESEMBLES A SPIDER-CRAB FROM THE MEDITERRANEAN (NATURAL SIZE)
THE GULF-WEED CRAB, _Planes minutus_ (SLIGHTLY ENLARGED)]
The little ”Gulf-weed Crab” (_Planesdrift-weed nearly everywhere throughout the telobe, and is especially coasso Sea, in mid-Atlantic It is occasionally drifted to the south coasts of the British Islands In Sloane's ”Natural History of Jamaica,” published in 1707-1725, it is stated of the Gulf-weed Crab that ”Colu in the sea, conceived hie he made on the discovery of the West Indies”
A few other Crustacea also form part of the peculiar fauna which is associated with the Sargasso weed, notably a swi Crab, _Neptunus sayi_, and two or three species of Prawns All of these are coloured olive-green, like the weed a which they live