Stages of Development of Lernæa branchialisFig. 73—Stages of Development ofLernæa branchialis.F is slightly, the Other Figures greatly, enlarged.(After A. Scott.)A, Nauplius, just hatched; B, young female taken from gills of Flounder; C, free-swimming stage of female, after leaving Flounder; D, free-swimming male; E, female just after settling on gills of Whiting; F, fully-developed female.View larger image
Fig. 73—Stages of Development ofLernæa branchialis.F is slightly, the Other Figures greatly, enlarged.(After A. Scott.)
A, Nauplius, just hatched; B, young female taken from gills of Flounder; C, free-swimming stage of female, after leaving Flounder; D, free-swimming male; E, female just after settling on gills of Whiting; F, fully-developed female.
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On the gills of Cod, Haddock, and other common fish, we often find a red worm-like parasite,Lernæabranchialis(Fig. 73, F), which at first sight seems to bear no sort of resemblance to a Crustacean. The soft body is curiously doubled up, and is attached to the host by a narrow neck; while dissection will reveal a small head buried in the flesh of the fish's gills, and having three branched outgrowths, which penetrate into the surrounding tissues and make the attachment of the parasite more secure. Near the hinder end of the body are two coiled threads, which are the egg-masses. The reduced mouth parts and the microscopic vestiges of the swimming feet may be detected on and near the head, but apart from these it would be hard to find any characters to show that the animal is a Crustacean.
The life-history ofLernæais very remarkable. The young are hatched in the nauplius stage (Fig. 73, A), and after passing through some further free-swimming stages they become parasitic on a fish. Curiously enough, however, they choose a very different host from that on which the adults are found, for at this stage (Fig. 73, B) they attach themselves to the gills of one of the Flat-fishes (Pleuronectidæ), such as the Flounder, Plaice, etc., attachment being effected by a frontal cement gland similar to that of the larval Caligidæ, already mentioned. The animal is now without the power of swimming, its appendages becoming reduced to stumps and losing their setæ. After passing sometime in this condition, the larva again acquires the power of swimming, and leaves its host. Both sexes become mature in this free-swimming stage (Fig. 73, C, D), and impregnation is effected. The males die without developing further, but the females seek a second host, a fish of the family Gadidæ, such as the Cod, Haddock, etc., and, settling on the gills, become metamorphosed (Fig. 73, E) into the adult form described above.
Within the gill cavities of the strange-looking fish known as the Angler or Fishing-frog (Lophius piscatorius) there may often be found specimens of another parasitic Copepod,Chondracanthus gibbosus. It has a soft, unsegmented body about half an inch long, provided with numerous blunt lobes which give it a very irregular shape. On the under-side, near the front, are forked lobes representing two pairs of the swimming feet. At the hinder end are usually attached a pair of long thread-like egg-masses. Just at the point where the egg-masses are attached, close inspection of the under-side of the body will reveal a very minute maggot-like object. This is a male individual, which is attached, like a secondary parasite, to the body of the enormously larger female.
Stages in the Life-history of Hæmocera danæFig. 74—Stages in the Life-history ofHæmocera danæ,One of the Monstrillidæ. (From Lankester's "Treatise on Zoology," after Malaquin.)A, Free-swimming nauplius larva; B, embryo after penetrating into the body of the wormSalmacina; C, D, E, successive stages in the body of the host; F, free-swimming adult female. (All greatly enlarged, not to same scale.)a′, Antennule;br, brain;e, nauplius eye;f, swimming feet;g.s., hairs on which the eggs are carried;m, position of mouth;md, hooked mandible of nauplius;n, nerve cord;ov, mass of eggs carried by female;ovy, ovary;pr, absorptive processes.View larger image
Fig. 74—Stages in the Life-history ofHæmocera danæ,One of the Monstrillidæ. (From Lankester's "Treatise on Zoology," after Malaquin.)
A, Free-swimming nauplius larva; B, embryo after penetrating into the body of the wormSalmacina; C, D, E, successive stages in the body of the host; F, free-swimming adult female. (All greatly enlarged, not to same scale.)a′, Antennule;br, brain;e, nauplius eye;f, swimming feet;g.s., hairs on which the eggs are carried;m, position of mouth;md, hooked mandible of nauplius;n, nerve cord;ov, mass of eggs carried by female;ovy, ovary;pr, absorptive processes.
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In all the cases mentioned, the animal is parasitic in the final state of its existence—at least in the female sex—but there are a few Copepoda which are free-swimming, both when young and when adult, but parasitic in the intermediate stages. Among the Copepoda takenby the tow-net in British seas, there may sometimes be found species of the family Monstrillidæ (Fig. 74, F), which are remarkable for having no appendages between the antennules and the first pair of swimming feet. They have no trace of jaws, and only a minute vestige of a mouth-opening; while internally there is no food-canal, so that the animals are incapable of taking nourishment. Their development was for long a mystery, but it is now known that the greater part of their life is passed as internal parasites in certain bristle-footed worms (Polychæta). The young are hatched as nauplius larvæ (Fig. 74, A) without mouth or food-canal, but capable of swimming, and having the third pair of appendages (mandibles) furnished with strong hooks, by means of which they fasten on to the worm which is to serve as their host. The nauplius bores through the skin of the worm, casting its cuticle and losing all its appendages in the process, and making its way into one of the bloodvessels in the form of a little oval mass of cells (Fig. 74, B), within which no organs except the degenerating nauplius eye can be detected. It later becomes enclosed in a delicate cuticle, and from one end two long finger-like processes grow out, which are believed to have the function of absorbing nourishment from the blood of the host (Fig. 74, C, D). Within the cuticle the organs of the adult animal are gradually differentiated (Fig. 74, E), and when fully formedit bores its way through the tissues of its host by means of rows of hook-like spines surrounding the pointed posterior end of the sac. On reaching the surface the enclosing membrane bursts, and the adult animal is set free.
Free-swimming Stages of Sacculina carciniFig. 75—Free-swimming Stages ofSacculina carcini.Much enlarged.(After Delage.)A, Nauplius; B, cypris stage.View larger image
Fig. 75—Free-swimming Stages ofSacculina carcini.Much enlarged.(After Delage.)
A, Nauplius; B, cypris stage.
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Early Stage of Sacculina within the Body of a CrabFig. 76—Early Stage ofSacculinawithin the Body of a Crab. (After G. Smith.)i, Intestine of the Crab;s, body of theSacculina, which afterwards emerges on the under-surface of the Crab's abdomen;r, roots of theSacculina.View larger image
Fig. 76—Early Stage ofSacculinawithin the Body of a Crab. (After G. Smith.)
i, Intestine of the Crab;s, body of theSacculina, which afterwards emerges on the under-surface of the Crab's abdomen;r, roots of theSacculina.
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Of all Crustacean parasites, however, perhaps the most remarkable in their structure and life-history are the Cirripedes of the order Rhizocephala. It is not uncommon on the British coasts to find specimens of the common Shore Crab or other Crabs which carry under the abdomen an oval fleshy body. This is the RhizocephalanSacculina carcini(Plate XXIX.), and it would hardly be possible to guess, from its appearance or structure, that it was a Cirripede or a Crustacean at all. It is attached to the under-side of the Crab's abdomen by a short stalk, and in the middle of its opposite surface is a small opening which leads into a cavity separating the outer "mantle" from the body of the animal. Very often this mantle cavity will be found to be full of eggs enclosed in sausage-shaped packets. At the point where the short stalk enters the abdomen of the Crab, it gives off an immense system of fine branching roots, which penetrate throughout the body of the Crab, and even into its legs and other appendages. By means of these roots theSacculinaabsorbs nourishment from the body-fluids of its host. Like most Cirripedes,Sacculinais hermaphrodite, and the body within the mantle cavity contains only thereproductive organs of the two sexes and a small nerve ganglion representing the whole of the nervous system. There is no mouth, no food-canal, and no trace of appendages. Another Rhizocephalan,Peltogaster, is not uncommonly found attached to the abdomen of Hermit Crabs. Although the nauplius larva ofSacculinawas described, and its resemblance to that of the Cirripedia pointed out, as long ago as 1836, by that acute observer, J. Vaughan Thompson, it is only recently that the full life-history has been made known by the researches of Professor Delageand Mr. Geoffrey Smith. The nauplius larva (Fig. 75, A) resembles that of the normal Cirripedes, especially in the shape of the dorsal shield, which is drawn out on either side in front into a pair of fronto-lateral horns. It has, however, no mouth, and the food-canal is quite absent. As in the normal Cirripedes, the nauplius is followed by acyprisstage (Fig. 75, B), also mouthless, and it is in this form that theSacculinaseeks the Crab on which it is to become parasitic. It would be almost impossible for thecyprislarva to settle on that part of the Crab where the adultSacculinais afterwards to appear, since the Crab usually has its abdomen closely pressed against the under-side of its thorax. The larva therefore attaches itself on some exposed part of the Crab, often on one of the legs, clinging to a hair by means of its antennules. It bores through the cuticle at the base of the hair, and the contents of its body pass into the interior of the Crab as a little mass of cells, the emptycyprisshell being cast off. This mass of cells, which constitutes the embryoSacculina, is carried about by the blood-currents of the Crab till it reaches the under-side of the intestine, where it becomes attached. It now begins to send out roots (Fig. 76), and as it grows the central mass travels backwards along the intestine of the Crab till it reaches the place where the adult parasite is to emerge. As the mass increases in size, and the organs of theSacculinabecome differentiated within it, its presencecauses the living tissues between it and the external cuticle to degenerate, so that when the Crab moults an opening is left through which the body of the parasite protrudes. Owing, no doubt, to the drain on its system due to the presence of theSacculina, the Crab ceases to grow, and it does not moult again as long as the parasite remains alive.
In addition to this arrest of growth,Sacculinaproduces in its hosts other changes, which affect chiefly the reproductive organs and the structures associated therewith. Crabs of either sex infected withSacculinaare incapable of breeding; the genital gland (ovary or testis) is found on dissection to be shrivelled up, and the external characters indicative of sex become strangely modified. The changes havebeen most fully studied in the case of a kind of Spider Crab common at Naples—Inachus mauritanicus. In this species it is found that females infected withSacculinashow no conspicuous external modification, except that the abdominal appendages, which in the normal females serve for the attachment of the eggs, are greatly reduced in size. Infected males, however, may assume to a greater or less degree the characters proper to the female sex. Some males show little change, except that the chelipeds remain small and flattened, as in the females and non-breeding males. Other specimens have, in addition, the abdomen much broader than in normal males, and sometimes as broad as in the females. Finally, some males develop on the abdomen, in addition to the rod-like appendages on the first and second somites, characteristic of the male sex, two-branched appendages on the next three somites, as in the females; these individuals are, in fact, so completely intermediate in character between the two sexes that it is only by dissection that it is possible to recognize them as modified males.
An indication of the way in which the degenerate Rhizocephala have been derived from normal Cirripedes is given by a peculiar species of pedunculate Barnacle,Anelasma squalicola, which lives attached to Sharks and Dogfish in the North Sea. InAnelasmathe peduncle becomes deeply buried in the flesh of the Shark, and its surface is covered with shortbranching, root-like filaments. As in the case of the Rhizocephala, these roots appear to absorb nutriment from the host, and, althoughAnelasmapossesses a food-canal and mouth, the cirri are reduced in size and devoid of hairs, so that they cannot be used for obtaining food as in ordinary Barnacles.
The Crustacea come into relation with human life in the most obvious and direct way in the case of those species that are used for food. The number of species so used in various parts of the world is very large, almost the only necessary condition being that the species shall be sufficiently large and abundant to make it worth while to fish for it.
As most of the larger Crustacea belong to the Decapoda, it is this order that supplies practically all the edible species, almost the only exceptions being a few Barnacles which are eaten in various parts of the world. Thus the sessile BarnacleBalanus psittacus, found on the coasts of Chili, and growing to a length of 9 inches by 2 or 3 inches diameter, is, according to statements quoted by Darwin, "universally esteemed as a delicious article of food," and the pedunculatePollicipes cornucopiais used for food on the coasts of Brittany and Spain.
By far the most valuable of all the edible Crustacea are the European and American Lobsters (HomarusgammarusandH. americanus). The former is found on the coasts of Europe from Norway to the Mediterranean, living mostly a short distance below low-water mark wherever the bottom is rocky. At some places, as for instance at Worthing, Lobsters are common on a sandy bottom, but as a rule they seem to prefer localities where the crevices of a rough hard bottom afford abundance of shelter. They are usually caught in traps known as "Lobster pots" or "creels," which vary in construction in different localities. In some cases they are made of wicker-work, hemispherical in shape, with a funnel-shaped opening on top, so devised as to permit the Lobsters to enter easily, while preventing their escape. Another form is semi-cylindrical, with a framework of wood covered with netting or with wooden spars, and having two funnel-shaped entrances at the sides. These traps are baited with pieces of fish, preferably stale, and are sunk in suitable places, each attached by a line to a buoy or float.
Important Lobster fisheries are carried on in Norway, Scotland, England, Ireland, Heligoland, and other parts of the coasts of Northern Europe. In the South the Lobster fishery is of less importance, other large Crustacea, especially the Spiny Lobster, being more abundant and more highly esteemed.
The American Lobster, as already mentioned, closely resembles the European species, the chief difference being in the form of the rostrum (seeFig. 9, p. 32). It is found on the Atlantic coast from Labrador to Cape Hatteras, but it is not abundant south of New Jersey. The canning of Lobsters is a very important industry in Newfoundland, the Maritime Provinces of Canada, and the Northern New England States.
The only other species of the genusHomarus(H. capensis) is found at the Cape of Good Hope, but it is of small size and is of no economic importance.
The European Lobster rarely reaches a weight of 10 pounds, although individuals of 14 pounds weight have been caught. In America, there are authentic records of Lobsters weighing 20 and even 23 pounds.
The bad effects of over-fishing have become apparent of late years, especially on the American coast, in the reduced average size of the Lobsters caught rather than in a diminution of the total yield of the fishery. Numerous experiments in legislation have been made with a view to checking the depletion of the fishing-grounds, but in no case with conspicuous success. A "close time" for the spawning Lobsters has often been tried, but the fact that the female carries the eggs attached to her body for nearly a year after spawning makes it quite impossible to give effective protection by this means. In most Lobster-fishing districts a minimum size is fixed by law, below which it is illegal to take or sell Lobsters, and in many cases also the capture offemales carrying spawn, or, as it is termed, "in berry," is prohibited.
Calappa flammeaPLATE XXXTHE "NORWAY LOBSTER,"Nephrops norvegicus,ABOUT ONE-THIRD NATURAL SIZE(From Brit. Mus. Guide)View larger image
PLATE XXX
THE "NORWAY LOBSTER,"Nephrops norvegicus,ABOUT ONE-THIRD NATURAL SIZE
(From Brit. Mus. Guide)
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The so-called "Norway Lobster" (Nephrops norvegicus—Plate XXX.), the "Dublin Prawn" of the London fishmongers, is a smaller and much less valuable species than the common Lobster. It may be recognized at once by its long and slender claws, furnished with rows of tubercles or blunt spines, and by the sculptured markings on the somites of the abdomen. When alive it is of an orange colour, beautifully marked with red and white. It differs considerably in its habits from the common Lobster, living at a considerably greater depth (30 to 60 fathoms in Norway), and on a muddy bottom. It is generally taken by trawling, and is captured in large quantities by trawlers fishing in various parts of the North Sea. Since it must be cooked soon after it is caught, and cannot easily be brought to market alive like the common Lobster, only a small number of those actually caught are made use of. Formerly most of those sold in London were caught in the Irish Sea (whence the name of "Dublin Prawn"), but the North Sea is now the chief source of supply. The species is found in suitable localities from Norway to the Mediterranean, and is especially abundant in the Adriatic, where it is caught and sold in Venice and elsewhere under the name of "Scampo."
The Spiny Lobster, Rock Lobster, or Sea-crawfish (Palinurus vulgaris—Plate V.), is common on thesouth and south-west coasts of the British Islands, becoming rare in the north, although specimens have been found as far north as Orkney, and there is a single record of the species from the West of Norway. It is far less commonly used for the table in this country than in France, where it is known as "Langouste" and is very highly esteemed.
Various species of Spiny Lobsters belonging to the same family (Palinuridæ) as the European species are found in different parts of the world. In tropical countries the species ofPanulirusare commonly used for food (for example,P. interruptusin California andP. fasciatusin India), as are species ofJasusin South Africa, Australia, and New Zealand. Recently a consignment of Spiny Lobsters (Jasus lalandii) was sent to the London market from the Cape, but it appears that the experiment was not altogether successful.
Belonging to the same tribe (Nephropsidea) as the Lobsters are the fresh-water Crayfishes. The English Crayfish (Astacus pallipes) is common in many rivers as far north as Lancashire, and in some parts of Ireland, but is not found in Scotland. It is not much esteemed for the table, and although small numbers are sent to Billingsgate, chiefly from Leicestershire, they are said to be used only for garnishing dishes. The same species occurs on the Continent of Europe, chiefly in the west and south (France, Germany, Switzerland, Spain, Italy, andthe Balkan Peninsula). It is known in France as "Écrevisse à pattes blanches" (from the whitish colour of the under-side of the large claws), and in Germany as "Steinkrebs," and is distinguished, among other characters, by the shape of the rostrum (Fig. 77, B), which has a tooth on each side close to the point. Far more important as an article of food is the largerAstacus fluviatilis, the "Écrevisse à pattes rouges" or "Edelkrebs," which is found in France, Germany, Austria, Southern Sweden, Russia, etc. In this species the under-side of the large claws is generally of a fine red colour, and the rostrum (Fig. 77, A) has a pair of side-teeth about the middle of its length, and a long slender point. The red-clawed Crayfish is an important article of commerce on the Continent, and is sent to the London market in considerable numbers, chiefly from Germany and South-West Russia. In France it is cultivated for the market in "Crayfish farms" on a large scale.
Rostrum and Fore Part of Carapaces of CrayfishFig. 77—Rostrum and Fore Part of Carapace, seen from Above, of (A) Red-clawed Crayfish(Astacus fluviatilis)and (B) White-clawed or English Crayfish(Astacus pallipes)
Fig. 77—Rostrum and Fore Part of Carapace, seen from Above, of (A) Red-clawed Crayfish(Astacus fluviatilis)and (B) White-clawed or English Crayfish(Astacus pallipes)
A species of Crayfish (A. leptodactylus) occurring inthe Lower Danube and in other rivers flowing into the Black Sea sometimes finds its way to the London market, although it is less valued than the red-clawed species. It is distinguished by its long and slender claws, by the spiny edges of the rostrum, and by other characters. A fourth species (A. torrentium), occurring chiefly in Central Europe, is very closely allied toA. pallipes, and, like it, is of little value for the table.
Within the last thirty years the Crayfish fisheries of Western Europe have suffered heavily from outbreaks of an epidemic disease which has all but exterminated these animals in certain districts. In this country it is said to be responsible for the almost complete disappearance of Crayfish from localities where they were formerly plentiful, as, for instance, in the neighbourhood of Oxford. The cause of the disease is believed to be a protozoan parasite belonging to the group Myxosporidia.
In other parts of the world it does not seem that the fresh-water Crayfishes are of much importance as an article of food. Some species ofCambarusare so used to a limited extent in the United States, and the giganticAstacopsis serratus(Plate XX.) is known as the "Murray River Lobster" in the markets of Sydney and Melbourne.
Crangon vulgarisFig. 78—The Common Shrimp(Crangon vulgaris).Natural SizeView larger image
Fig. 78—The Common Shrimp(Crangon vulgaris).Natural Size
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The Decapods of the suborder Natantia comprise a large number of edible species, generally known as Shrimps and Prawns. The Common Shrimp,Crangon vulgaris(Fig. 78), which is plentiful on the British coasts wherever the bottom is sandy, is about two or three inches long, and when alive is of a translucent greyish colour speckled with brown. It differs from most of the Natantia in having the body somewhat flattened from above downwards, and the rostrum very short. When boiled, it is of a reddish-brown colour, and from this it is sometimes known as the "Brown Shrimp." On many parts of the coast the Shrimp fishery is of considerable importance. Most often the Shrimps are caught by means of a large bag-net attached to a semicircular hoop with a long handle, and pushed over the surfaceof the sand by a fisherman wading in the water at ebb-tide.
A variety of species are sold in England under the name of Prawns. The largest of the native species, to which the name of Common Prawn is perhaps most properly restricted, isLeander serratus. It grows to a length of over 4 inches, and has a long serrated rostrum extending beyond the antennal scales and curving upwards at the point. The first and second pairs of legs end in small pincer-claws. When alive the animal is very transparent, and beautifully marked with bands of brown and red on the body and limbs. A smaller species of the same genus (L. squilla), distinguished by the much shorter and straighter rostrum, and another very similar species of which the proper name appears to beL. adspersus(often known asL. fabricii), are said to be sold on some parts of the English coast as "Cup Shrimps."
Much commoner, at least in the London market, than the species ofLeanderisPandalus montagui, often sold under the general name of Prawn, but sometimes called the "Pink Shrimp." This resemblesLeander serratusin having a long, serrated, up-curved rostrum, but differs from it strikingly in the form of the anterior pairs of feet. The first pair appear to the naked eye to have no pincer-claws, but to end in a sharp point, resembling the third maxillipeds, which are just in front of them. As a matter offact, they do have pincers, but so minute that they can only be detected by microscopic examination. The feet of the second pair are unequal in length on the two sides, that on the left side being the longer, and are very slender. They end in small pincers, and examination with a pocket-lens will show that the carpus, or "wrist," and the segment below it (merus) are broken up into a large number of short segments, so that the limb is extremely flexible. When alive, the animal is even more handsomely marked than the Common Prawn.
Pandalus borealisFig. 79—The Norwegian Deep-water Prawn(Pandalus borealis),Female. (After Sars.)The second leg of the right side is indicated by dotted lines.View larger image
Fig. 79—The Norwegian Deep-water Prawn(Pandalus borealis),Female. (After Sars.)
The second leg of the right side is indicated by dotted lines.
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A large species of Prawn is now imported to this country in considerable quantities from Norway. This isPandalus borealis(Fig. 79), a species closely allied to the last-named, but differing in the longer and more slender rostrum and in many other characters, as well as in its larger size (specimenshave been recorded of 6 inches in total length). It also differs in its habitat, for whileP. montaguilives in shallow water, or even between tide-marks,P. borealisoccurs at depths of 30 to 60 fathoms in the Norwegian fjords. The recent development of the fishery forP. borealisin Norway is a striking example of the practical value of zoological research. Until 1898 the species was hardly known except to zoologists, although a small fishery was carried on in the Drammen Fjord, near Christiania. The investigations of the naturalists employed by the Norwegian Department of Fisheries showed that the species existed in vast numbers in the deeper water of many of the fjords, and that it could be captured in abundance by means of a suitably-devised trawl-net. As a result, a very profitable fishery was established, and the "deep-water Prawns" are now not only largely consumed in Norway, but are exported in increasing quantities to the English and other markets.
In the warmer seas the large Prawns of the genusPenæusare of considerable importance. Thus, in the Mediterranean countries,Penæus caramote(Plate IV.) is highly esteemed for food, andP. setiferandP. brasiliensisare largely consumed in the Southern United States.P. monodonand other species are eaten in India. An attempt has been made to send a species of the same genus (apparentlyP. indicus) in a frozen state from Queensland to the London market.
Numerous other species of Natantia are used for food in various parts of the world, but the only ones that need be further mentioned here are the River Prawns of the genusPalæmon, which are abundant in the fresh waters of most tropical countries, and sometimes grow to a very large size. They are generally distinguished by the fact that the legs of the second pair are very long, forming powerful pincer-claws. In the West Indies and Central America,P. jamaicensis(Plate XXI.), which reaches a length of 10 inches exclusive of the great claws, is sold in the markets, while in India and elsewhere in the EastP. carcinus, which grows to an even greater size, and other smaller species, are used for food. The fresh-water Prawns of the family Atyidæ, on account of their small size, are not of much importance from this point of view, but Professor Hickson states that the littleCaridina nilotica, a very widely-distributed species, is eaten in Celebes.
Cancer pagurusPLATE XXXITHE COMMON EDIBLE CRAB,Cancer pagurus.BRITISH.(MUCH REDUCED)View larger image
PLATE XXXI
THE COMMON EDIBLE CRAB,Cancer pagurus.BRITISH.(MUCH REDUCED)
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Among British Crustacea, the next in importance to the Lobster as an article of food is the Edible Crab,Cancer pagurus(Plate XXXI.), known in Scotland as the "Partan." Like the Lobster, it is found on rocky coasts in shallow water, and young specimens are often taken between tide-marks. It grows to a size of more than 10 inches across the shell, and may reach a weight of 12 pounds. The means used for its capture are the same as in the case of the Lobster, and the fishery is of considerableimportance on many parts of the British coasts. On the other hand, a Connemara fisherman, who was using these Crabs for bait, received with incredulity the statement that they were good for human food!
The Shore Crab,Carcinus mænas(Plate IX.), is not of much importance as food in this country, although it is recorded that fifty years ago great numbers were brought to the London market. On the shores of the Mediterranean and Adriatic, however, and especially in Venice, this species is regarded as a delicacy, particularly in the soft-shelled state after moulting.
On the Atlantic coast of North America, the most important edible Crustacean after the Lobster is the "Blue Crab" (Callinectes sapidus), one of the Swimming Crabs (Portunidæ). This is consumed in large quantities, especially in the soft-shelled state. Several other species of Crabs are eaten in America, including the little "Oyster Crab," a species ofPinnotheresliving in the American Oyster. From its small size, and the difficulty of obtaining it in numbers, it is a very costly delicacy.
In the East Indies the most important edible Crabs are various species of Portunidæ, especially the largeScylla serrataandNeptunus pelagicus.
Except as food, the Crustacea are of very little direct use to man. Almost the only instance in which they are otherwise utilized is in the case ofa species of sessile Barnacle (Balanus) which is cultivated in Japan for use as manure. The method of culture has been described by Professor Mitsukuri. Bunches of bamboo "collectors," like those used for the collection of oyster-spat, are fixed into the ground on tidal flats. After two or three months they are taken up, and the Barnacles with which they have become covered are beaten off and sold for use as manure.
Apart from their direct utility, however, the Crustacea are indirectly of great importance as providing a large part of the food-supply of marketable fishes. From this point of view, a study of the habits and distribution of the commoner species may be of practical value in throwing light on the migrations and other obscure points in the life-history of the fishes that prey upon them. As an example of this, we may refer to some investigations on the Mackerel fishery recently carried out by the naturalists of the Marine Biological Association at Plymouth. In the spring and early summer months the Mackerel migrate into inshore waters for the purpose of spawning. During this period the fish congregate in shoals at the surface of the sea, and are captured in drift-nets. The extent of this "shoaling" varies greatly from year to year, and determines whether the season shall be a profitable one for the fishermen or not. When shoaling, the fish feed exclusively on plankton, consisting largely of Copepoda, and it hasbeen shown by Mr. G. E. Bullen that the fluctuations in the yield of the Mackerel fishery from year to year follow very closely the fluctuations in the abundance of the Copepod plankton on the fishing-grounds. The investigation has been carried a step farther by Dr. E. J. Allen, who points out that the abundance of Copepods is determined by the abundance of the Diatoms and other minute vegetable organisms of the plankton. These organisms are very largely influenced by the amount of sunshine during the period of their development in the earlier months of the year. Dr. Allen gives a diagram showing for each of seven years (1902-1908) the average number of hours of bright sunshine during the months of February and March in the South-West of England. With this he compares the number of fish caught in the month of May in each of these years by certain vessels engaged in the western Mackerel fishery. The correspondence between the two is very striking indeed, and justifies his conclusion that the amount of sunshine in the early months of the year determines the abundance of the vegetable life of the plankton, and through it of the Copepods and other animals which form the bulk of the plankton a little later in the year; and although there are doubtless other influences at work determining the success or failure of the fishery, it is largely a matter of the richness or poverty of the plankton harvest.
None of the Crustacea can be regarded as directly harmful to man. They have not the power of inflicting envenomed wounds which renders some other Arthropods, such as Scorpions, some Spiders, Centipedes, and Insects, formidable in spite of their small size; and although blood-curdling tales of the ferocity of the Land Crabs are to be found in the accounts of old voyages, even the largest of these is hardly an antagonist to be dreaded.
A considerable number of invertebrate animals, not of themselves noxious, are now known to be the indirect cause of much serious injury to human life by harbouring and disseminating organisms which produce disease. The progress of research is adding, almost every day, to the number of species known to be disease-carriers, and it is possible that in the future some Crustacea as yet unsuspected may be added to the list.
The GribbleFig. 80—The Gribble(Limnoria lignorum).Much enlarged(From British Museum Guide, after Sars.)View larger image
Fig. 80—The Gribble(Limnoria lignorum).Much enlarged(From British Museum Guide, after Sars.)
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At present, however, there is only one case in which a Crustacean has been shown to be concerned in the transmission of a parasite of man. The "Guinea-worm,"Filaria(orDracunculus)medinensis, is a parasite belonging to the group of "Thread-worms" (Nematoda) which causes dangerous abscesses under the skin of the legs in many parts of tropical Africa. It has been shown that the embryos of the worm, which are discharged in vast numbers on the bursting of the abscess, do not develop unless they fall into water containing certainspecies of the CopepodCyclops(seeFig. 14, p. 39). In some way not yet understood, the embryos penetrate into the body cavity of theCyclops, where they undergo a metamorphosis. For their further development it is necessary that theCyclopsshould be swallowed by man, as may easily happen in drinking water from a pond. When theCyclopsis digested the larval worms are set free, and they bore their way through the tissues of their human host till they reach the place (generally under the skin of the leg) where they complete their development and produce the innumerable embryos that are set free in the way just described.
A few Crustacea inflict a certain amount of injury on man in more indirect ways. In tropical countries, Land Crabs are often troublesome in gardens, and may cause serious damage to young plants in sugar-cane plantations and rice-fields. In gardens in this country, the Woodlice, as already mentioned, are sometimes destructive to seedlings and delicate plants. The little fresh-water Isopod,Asellus aquaticus, is accused of destroying the nets used in fishing for Pollan in Lough Neagh in Ireland.
Probably the most important of all Crustacea, however, as regards their destructive activity, are the species which bore into wood, and sometimes do extensive damage to the submerged timber of piers, jetties, and similar structures. On our own coast the most destructive is a little Isopod known as the "Gribble"(Limnoria lignorum—Fig. 80), which is distributed from Norway to the Black Sea, and occurs also on the Atlantic coast of North America. Several species of the same genus having similar habits are found in other parts of the world. The Gribble was first discovered as a British species by Robert Stevenson, the celebrated lighthouse engineer, who found it in 1811 destroying the woodwork employed in the erection of the Bell Rock Lighthouse, and sent specimens to Dr. Leach of the British Museum. The animal is only about one-eighth of an inch in length, and its cylindrical burrow is about one-fifteenth of an inch in diameter, and penetrates for a depth of one or two inches. The excavation of the wood is effected by means of the mandibles, which are unusually strong; and when the animals arenumerous the burrows are driven so close together that the surface of the wood is reduced to a spongy mass which is rapidly washed away by the waves (Plate XXXII.). The Gribble is often accompanied by another Crustacean of similar habits, the AmphipodChelura terebrans. The latter is about one-fifth of an inch in length, and differs from most Amphipods in having the body somewhat flattened from above downwards instead of from side to side. The burrows made byCheluraare shallower than those of the Gribble, and generally run more or less parallel to the surface of the wood.
Piece of timber from ryde pier showing damage caused by Limnoria and CheluraPLATE XXXIIPIECE OF TIMBER FROM RYDE PIER SHOWING DAMAGE CAUSED BYLimnoriaANDChelura(From Brit. Mus. Guide)View larger image
PLATE XXXII
PIECE OF TIMBER FROM RYDE PIER SHOWING DAMAGE CAUSED BYLimnoriaANDChelura
(From Brit. Mus. Guide)
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Since the acceptance by naturalists of the theory of Evolution as indicating the mode of origin of the various forms of life now existing, one of the chief lines of biological investigation has had for its object the reconstruction of the pedigree (or, as it is called, the "phylogeny") of the larger groups of the animal and vegetable kingdoms. In attempting to do this, there are three main sources from which evidence may be drawn. The results of Comparative Anatomy enable us to decide with more or less confidence as to the degrees of relationship between the groups of organisms, and to distinguish between the more primitive and the more specialized; the study of Embryology is, at least, an indispensable adjunct to Comparative Anatomy, even if it does not, as was once supposed, give us an actual recapitulation of ancestral history; and, finally, the study of Fossil Remains holds out the hope that we may be able to find the ancestral types themselves.
It is clear that evidence from the last-namedsource, when it is available, is the most important of all, since the order of succession of the various types is given by that of the rock strata in which they occur, and we can be quite certain that we are dealing, if not with the actual ancestors, at least with the forerunners of existing species. The "imperfection of the geological record," however, is so great that the organisms preserved in the fossil state represent only an insignificant part of the whole number of organisms that have lived on the globe since life began; and it is not surprising, therefore, that in many groups the study of fossils has hitherto afforded little help towards the working out of their genealogical history. Thus, among Crustacea there are many important groups such as the Copepoda, which are entirely unknown as fossils, their small and delicate bodies being ill adapted for preservation, although there is every reason to suppose that they are a very primitive and very ancient group. In many fossil Crustacea only the hard shell or carapace has been preserved, the appendages being lost or represented only by indecipherable fragments, and in some cases it is hardly possible to guess at the affinities of the animals. Further, several important groups are already represented in some of the oldest of the fossil-bearing rocks at present known, and the differentiation of these groups must have taken place in the dark ages before the record of the fossilsbegins. In spite of these disadvantages, however, the study of fossil Crustacea does throw considerable light on the evolution of the group, and it is likely that interesting results in this direction await future investigations.