Restoration of a TrilobiteFig. 81—Restoration of a Trilobite(Triarthrus becki),showing the Appendages. Upper Side on Right, Under-side on Left. Slightly enlarged.(After Beecher.)View larger image
Fig. 81—Restoration of a Trilobite(Triarthrus becki),showing the Appendages. Upper Side on Right, Under-side on Left. Slightly enlarged.(After Beecher.)
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In the earliest fossiliferous rocks the most abundant and important Arthropods are the Trilobites (Fig. 81), an extinct group which appears to have been related to the primitive Crustacea. The name Trilobite refers to the three-lobed form of the body when seen from the dorsal side, most species having a pair of grooves running lengthwise which divide off a middle lobe containing the principal organs of the body from two lateral "pleural" expansions covering the limbs.The head-shield shows indications of being composed of five segments, and bears a pair of sessile compound eyes. It is followed by a number (up to twenty-six) of free somites, and the body ends in a tail-shield, or "pygidium," which is often plainly composed of several somites fused together. Although Trilobites are among the commonest and most familiar of fossils in the older rocks, the nature of their appendages remained quite unknown until within recent years, when specimens of several species showing the structure of the limbs and under-side of the body were discovered in America. From these it appears that the head bore in front a pair of long thread-like antennæ and four pairs of two-branched appendages, each with a jaw process, or "gnathobase," turned towards the mouth, which is covered below by a large anterior lip, or "hypostome." It seems probable that the five pairs of head-appendages correspond respectively to the antennules, antennæ, mandibles, maxillulæ, and maxillæ, of Crustacea; but the second pair appear to have acted as jaws, retaining the gnathobase which, among Crustacea, is only hinted at by the hooked spine on the antenna of the nauplius larva.
Each of the free somites and of those forming the tail-shield bears a pair of two-branched appendages, not differing greatly from the posterior appendages of the head, but becoming smaller and more flattened towards the hinder end of the body. The numerousgenera and species of Trilobites present great differences in the form and ornamentation of the dorsal surface of the body, and it is probable that considerable differences may also have existed in the structure of the limbs, which are only known in two or three species. Some Trilobites are among the most ancient of known fossils, being found in rocks of the Lower Cambrian epoch. The group reaches its maximum development in the Ordovician, and the number of the species and size of the individuals gradually diminish through the Silurian and Devonian till they become extinct at the close of the Carboniferous epoch, except for a single species found in rocks of Permian age in America.
Although zoologists are not all agreed as to the precise systematic place to be assigned to the Trilobites, there can be little doubt that they were related more or less closely to the most primitive Crustacea, and they are of special interest as preserving for us the stage in which the second pair of appendages were still used as biting jaws, and had not moved forwards in front of the mouth to become antennæ, as in all living Crustacea.
Contemporary with some of the earliest Trilobites, however, are undoubted Crustacea, which, so far as we know their structure, are not very different from types now living. In the Cambrian epoch the Branchiopoda appear to be represented byProtocaris, which in its general form resemblesApus; and there are a variety of genera andspecies of Ostracoda, although, since their shells alone are preserved, it is not possible to determine their exact relations to existing forms. In the succeeding Ordovician and Silurian epochs we first meet with the remains of Barnacles, and it is interesting to note that some of them are referred to the generaPollicipesandScalpellum, which are represented by numerous species in the seas of the present day. Along with these, however, are some strange-looking forms (Turrilepas, etc.), having the body covered with rows of overlapping plates. If these are really Cirripedes, they must have differed considerably in structure from the more modern types.
Ceratiocaris papilioFig. 82—Ceratiocaris papilio,One of the Fossil Phyllocarida. (From Lankester's "Treatise on Zoology," after H. Woodward.)a, Traces of antennules;m, possibly mandibles;r, rostral plateView larger image
Fig. 82—Ceratiocaris papilio,One of the Fossil Phyllocarida. (From Lankester's "Treatise on Zoology," after H. Woodward.)
a, Traces of antennules;m, possibly mandibles;r, rostral plate
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Pygocephalus cooperiFig. 83—Pygocephalus cooperi,from the Coal-measures: Under-Side of a Female Specimen, showing the Overlapping Plates of the Brood-pouch.(From Lankester's "Treatise on Zoology," after H. Woodward.)View larger image
Fig. 83—Pygocephalus cooperi,from the Coal-measures: Under-Side of a Female Specimen, showing the Overlapping Plates of the Brood-pouch.(From Lankester's "Treatise on Zoology," after H. Woodward.)
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The Malacostraca are more interesting from the point of view of palæontology than the other subclasses of Crustacea, since the evolution of the group appears to have taken place within the period covered by the fossil records, and it is possible to trace the course of that evolution—at least, in its broad outlines. It has already been pointed out that the most primitive of existing Malacostraca are the Phyllocarida (Nebaliaand its allies), which are in several respects intermediate between the higher Malacostraca and the Branchiopoda; and it is interesting to find that fossils apparently belonging to the Phyllocarida are found far earlier than any of the other Malacostraca. In the Cambrian, and more abundantly in the Ordovician and Silurian, there are found Crustacea(Fig. 82) that resembleNebaliain having a large bivalved carapace, with a movable beak-like plate in front, a projecting abdomen without conspicuous limbs, and a pair of large spines at the sides of the telson. Unfortunately, we have almost no knowledge of the structure of the limbs; but it can hardly be doubted that these very ancient Crustacea were allied to the existing Phyllocarida, and that they included the forerunners of the higher Malacostraca.
It is in the Carboniferous epoch, in all probability, that we must look for the origin of most of the existing orders of Malacostraca. In the rocks of this age in different parts of the world there have been found a number of undoubted Malacostraca, nearly all of the shrimp-like form which there is good reason to believe to be a primitive characteristic.Some of these (Pygocephalus—Fig. 83) have recently been shown to possess a brood-pouch formed of overlapping plates on the under-side of the thorax, and thus resemble the existing Mysidacea, which stand at the base of the Peracaridan series of orders. Others have a pair of strong side-spines near the tip of the telson, and in other ways resemble the recent Euphausiacea, so that they may have been primitive members of the Eucaridan series.
The Tasmanian Mountain ShrimpFig. 84—The Tasmanian "Mountain Shrimp"(Anaspides tasmaniæ),a Living Representative of the Syncarida. Slightly enlargedc.gr., "Cervical groove," marking off the first thoracic somite; ii-viii, the remaining thoracic somites; 1-6, the abdominal somitesView larger image
Fig. 84—The Tasmanian "Mountain Shrimp"(Anaspides tasmaniæ),a Living Representative of the Syncarida. Slightly enlarged
c.gr., "Cervical groove," marking off the first thoracic somite; ii-viii, the remaining thoracic somites; 1-6, the abdominal somites
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Among the Crustacea of the Carboniferous and Permian epochs, there are a number of forms of which the affinities were until recently quite obscure. They have two-branched antennules, a scale-like exopodite on the antenna, and the last pair of appendages (uropods) form, with the telson,a tail-fan. In these points they resemble the shrimp-like forms, but there is no carapace, and all the somites of the thorax are distinct, so that the form of the body is rather that of an Amphipod or Isopod. On the discovery of the remarkable CrustaceanAnaspides(Fig. 84), which lives in fresh-water pools in the mountains of Tasmania, it was pointed out that it agreed with the fossil generaUronectes,Palæocaris, and their allies, in those very characters in which they differed from all other Crustacea, and that it must be regarded as a surviving representative of the ancient group to which the name of Syncarida had been given. The more recent discoveries of living forms,Paranaspidesfrom the Great Lake of Tasmania andKoonungafrom fresh-water pools nearMelbourne, and of the fossilPræanaspides(Fig. 85) from the Coal-measures of Derbyshire, have tended to support this conclusion. There can be little doubt that the Syncarida arose during the Carboniferous epoch (or earlier) from primitive shrimp-like forms which lost the carapace; but, after flourishing for a relatively brief period, the group dwindled away, although a few survivors have lingered on, like so many other "living fossils," in the isolated Australian region.
Præanaspides præcursorFig. 85—Præanaspides præcursor,One of the Fossil Syncarida, from the Coal-measures of Derbyshire. Slightly enlarged. (After H. Woodward.)View larger image
Fig. 85—Præanaspides præcursor,One of the Fossil Syncarida, from the Coal-measures of Derbyshire. Slightly enlarged. (After H. Woodward.)
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It must be pointed out that, in spite of the resemblance of the body ofAnaspidesto that of an Amphipod, the Syncarida can have had no close relation to the origin of the Isopoda and Amphipoda. These have also been derived from a shrimp-like type, but their possession of a brood-pouch, amongother characters, shows that they are linked on to the Mysidacea, and must have arisen from some primitive member of that group, likePygocephalus. Although palæontology as yet gives little help in tracing the course of their evolution, we can imagine what the intermediate links must have been like by comparison with the living Cumacea and Tanaidacea.
It is possible, indeed, that the divergence of the Isopod line of descent from that of the Mysidacea took place earlier than the Carboniferous epoch, for there has recently been discovered in rocks of Devonian age in Ireland a single specimen of a fossil, to which the name ofOxyuropodahas been given, which has every appearance of being an Isopod. At all events, undoubted Isopods make their appearance in rocks of the Secondary Period, and some of those from the Jurassic epoch are not very different in general form from types still existing.
Some of the Carboniferous shrimp-like Crustacea present characters which seem to point in the direction of the Stomatopoda, and fossils which clearly belong to that group are found in Jurassic and later deposits. In the Cretaceous epoch there were Stomatopoda resembling modern types so closely that they have been referred to the existing genusSquilla. We are even able to say that they resembled the living Stomatopoda in their mode of development, for larvæ of the type known asErichthushave beenrecognized in rocks of Cretaceous age from Lebanon. This is a striking example of the way in which, by a fortunate accident as it were, organisms apparently ill-adapted for fossilization may occasionally be preserved.
Of the Decapoda the geological history is tolerably full, and it is possible to trace in its broad outlines the course of evolution of the various suborders. Here again it is likely that the beginnings of the group are to be sought for in the Carboniferous epoch, and some of the obscure shrimp-like forms of that age show hints of an affinity with the Decapods. In the Triassic epoch, however, and more abundantly in the succeeding Jurassic, a number of types are found which seem to include primitive representatives of several of the existing groups of Natantia and Reptantia. It is noteworthy that among them are some forms (Æger, etc.) resembling the existing Stenopidea, a tribe which in some respects is intermediate between these two suborders. In the Stenopidea the first three pairs of legs bear pincer-claws, as in the Lobster, but the third pair is much the largest; andÆgeragrees with them in this unusual character, though there is little else, in what is known of its structure, to help to determine its affinities.
The tribe Penæidea, which occupies in many respects a primitive place among the Natantia, is abundantly represented in the Jurassic epoch,especially in the lithographic stone (Upper Jurassic) of Solenhofen, and by somewhat doubtful specimens from the earlier Trias. All these agree in having the first three pairs of legs with pincer-claws, and not differing greatly in size. Some of the Jurassic and later fossils are of so modern a type that they have been referred to the existing genusPenæus.
The Upper Jurassic rocks also preserve the earliest undoubted specimens of true Prawns of the tribe Caridea, and some of these show swimming branches (exopodites) on the thoracic legs, so that they were probably related to the primitive family Acanthephyridæ, of which the existing members are found in the deep sea. It is possible, however, that Caridea were already in existence far earlier, for some of the obscure Carboniferous forms seem to have the broadened side-plates of the second abdominal somite, which, so far as we know, are exclusively characteristic of that tribe.
The Reptantia, forming the other large division of the Decapoda, also had their origin at least as early as the Triassic epoch, where representatives of the tribes Eryonidea and Scyllaridea are found. The history of the Eryonidea has already been discussed (p. 133) in dealing with the deep-sea Crustacea. The oldest representatives of the Scyllaridea belong to a family (Glyphæidæ) now wholly extinct, and are in many respects more primitive and lobster-like than any of the living Spiny Lobsters and their allies(Palinuridæ and Scyllaridæ). Forms with greatly thickened antennæ, indicating a transition to the Palinuridæ, begin to appear in the Jurassic; and in the later Cretaceous a genus,Podocrates, occurs which is hardly to be distinguished fromLinuparus, now living in Japanese seas. The Scyllaridæ have the antennæ modified into broad shovel-like plates, and perhaps take their origin fromCancrinus, in the Solenhofen lithographic stone (Jurassic), which has broad and apparently flattened antennæ. True Scyllaridæ are certainly found in Cretaceous deposits, and some, from the Upper Chalk, are even referred to the existing genusScyllarus.
The Anomura are almost unknown as fossils, but the true Crabs, or Brachyura, are abundantly represented. They first appear about the middle of the Jurassic epoch, and, as already pointed out, the earliest forms (Prosoponidæ) are referred to the Dromiacea, and appear to be closely related to the primitive Homolodromiidæ now living in the deep sea (p. 134). One of the oldest, and at the same time one of the most completely known, isProtocarcinus, from the Great Oolite of Wiltshire, which is preserved (in the only known specimen) with the abdomen partly extended, possibly indicating that the abdomen was less closely doubled under the body than in modern Crabs.
The next group of Crabs to appear are the Oxystomata, which are found from the middle of theCretaceous epoch onwards. The Brachyrhyncha perhaps begin to appear about the same time, but the affinities of the earlier types are doubtful, and it is only in the Tertiary that they become abundant and unmistakable. Several living genera, such asCancer, date back to the Eocene. The Spider Crabs (Oxyrhyncha) are rare as fossils, and the earliest specimens are found near the beginning of the Tertiary.
It may be useful to give here a few hints as to the methods of collecting Crustacea. Of the species that live in the sea, many may be found between tide-marks by turning over stones and searching among sea-weeds and in rock crevices. A small hand-net, made by fastening a bag of coarse muslin to a stout wire ring of a few inches diameter, is useful for fishing in rock pools. Shore-collecting in this manner is most productive at spring-tides, when the deeper levels of the shore are open to exploration.
Many burrowing species are to be found by digging in the sand near low-water mark. In addition to Crabs and other large species, many minute forms, Amphipods, Cumacea, and the like, inhabit such localities. The best way of collecting these is to stir up a spadeful of the sand in a bucket of water, and, after allowing the sand to settle for a few seconds, to pour off the water through a muslin bag. After repeating the operation two or three times, the contents of the bag are washed out into a jar or dishof sea-water for examination with a lens or under the microscope.
Dredging is the most effective method of obtaining Crustacea that live in deeper water. The dredge usually employed by naturalists consists of a heavy rectangular iron frame to which is attached a strong bag-shaped net. The two longer sides of the frame are sharp-edged and bevelled outwards, so as to "bite" when the dredge is dragged along the sea-bottom. To the shorter sides are hinged a pair of arms ending in rings. The dredge-rope is made fast to one of these rings, while the other is held only by a "stopping" of yarn, which gives way if the dredge should catch on a rock, and permits it to be dragged sideways off the obstruction. The size and weight of the dredge may vary according to the depth at which it is to be used and the power available for working it. A convenient size for use with a small sailing boat at moderate depths has a frame 20 by 5 inches.
Apart from dredging, many specimens from moderately deep water may be picked out from among the "rubbish" brought up on fishermen's lines or by the trawl, and various Crustacea besides the edible species find their way into Lobster and Crab pots. The true deep-sea fauna is, for the most part, only to be reached by specially-equipped expeditions, although specimens from great depths are occasionally obtained during the operations for the repair of submarine telegraph cables.
The floating animals of the surface of the sea areto be captured by means of the tow-net. This consists of a conical bag of muslin, cheese-cloth, or, best of all, silk "bolting-cloth," attached to a galvanized iron-ring of one or two feet in diameter, and having a zinc can or a strong glass jar fixed to the narrow end. The ring is attached by three equidistant cords to the towing line, and the net is towed slowly at or near the surface of the sea. When taken on board, the contents of the can are emptied into a jar of sea-water for examination. The tow-net is best used when there is only enough way on the boat to keep the net from sinking; if towed more rapidly, delicate organisms are apt to be crushed by the pressure of the water, or the net itself may be burst. The use of unnecessarily fine nets should be avoided. A fine-meshed net may not capture a single specimen of the larger Crustacea, even though these may be swarming in the water through which it is drawn.
By weighting the tow-net it may be used at various depths to capture the floating animals of mid-water. When it is so used, however, it is impossible to tell from what depth any particular specimen may have come, since it may have been captured during the hauling in of the net. For more precise investigations in deep water, "closing tow-nets" of various types have been devised, which can be opened by a "messenger" sent down the line when the net has reached the desired depth, and closed again by another "messenger" before it is hauled in.
A simple method that has proved very successfulfor collecting small Crustacea living on a sandy bottom in shallow water is to employ a light tow-net with a cane ring, and with a heavy sinker attached to the towing line at a distance of a few feet in front of the net. As the sinker is dragged along the bottom, the net floats up behind it, and catches small animals stirred up by its passage.
For collecting the smaller fresh-water Crustacea—Water-fleas and the like—a small muslin ring-net may be used in ponds and ditches. The plankton of the open water of lakes is best obtained by means of a tow-net like that described above for use in the sea.
The interesting blind species known as "Well Shrimps" are to be looked for in the water of springs and wells. In wells fitted with a pump, Professor Chilton found that "the Crustacea are often brought up most abundantly when pumping is first commenced, and that jerking the handle of the pump somewhat violently is often more successful than pumping at the ordinary rate." In disused open wells, they may be trapped by baiting a muslin ring-net with a piece of stale meat or fish, and pulling it up rapidly after it has remained in the well for a few hours. The subterranean waters of caves have yielded many curious species in various parts of the world. For the capture of species living in the deep water of large lakes, a special form of dredge has been devised with runners to prevent it from sinking into the soft mud, while the mouth of the net is raised a few inches above the bottom.
For preserving Crustacea the best medium to use is 70 per cent. alcohol. Strong spirit diluted with a little less than one-third its bulk of water gives about the required strength. If too strong spirit is used, the specimens tend to be hard and brittle, and delicate organisms become shrivelled. Methylated spirit as sold in the shops in this country contains mineral naphtha, and turns milky when water is added, so that it is unsuitable for preserving specimens. Methylated alcohol without naphtha can be bought, by permission of the Inland Revenue authorities, but only in considerable quantities at a time.
Formalin is very cheap and readily obtained, but it is much less suitable than spirit for most Crustacea, as it tends to make them stiff and fragile, and small forms containing much lime, such as Cumacea, may become decalcified. For Crustacea collected by the tow-net, however, formalin gives good results. A few drops of strong formalin, added to the water into which the tow-net has been washed, kills the animals in a few minutes. After they have sunk to the bottom, the liquid may be poured off and replaced by formalin diluted with sea-water (for marine plankton), or by a mixture of formalin and spirit. The most suitable strength of formalin varies with different organisms, but 5 per cent. (i.e., 1 part of commercial formalin to 19 parts of water) is perhaps most generally useful.
Crabs, Prawns, and the like, if put alive into strong spirit, may throw off some of their limbs,or else become so rigid that these break on the slightest manipulation. This may often be avoided by killing the animals in weak spirit (30 per cent. or less) before preserving in strong spirit. Marine species may also be killed by placing them in fresh water, care being taken not to allow them to remain in it longer than is necessary, as it causes distortion of the membranous appendages.
The larger Crabs, Lobsters, and the like, may be preserved dry, although in this state they are unsuitable for examination of the more delicate appendages. The carapace should be detached, and the soft parts cleaned away as far as possible, a bent wire being used, if necessary, to remove the flesh from the legs. The specimens should be dried in the shade, to preserve as much as possible of the natural colour.
With specimens intended for permanent preservation in spirit, the use of corks should be avoided, as they discolour the spirit, and ultimately the specimens. Small specimens are most conveniently kept in glass tubes closed with a piece of clean elder-pith (not cotton-wool), and placed, upside down, in a bottle of spirit. Labels to be placed inside the tubes are best written with Indian ink, and allowed to dry before immersion in the spirit.
The literature of Carcinology is bewildering in its extent, and is for the most part scattered through the volumes of scientific periodicals and the publications of learned societies in most of the languages of Europe. A guide to the current literature is provided by theZoological Record, the latest volume of which, relating to the year 1909, enumerates no fewer than 337 papers dealing wholly or in part with this group of animals.
The following short list of books in the English language may be of some help to the beginner. Most of them give references to the literature which will provide the necessary guidance towards a further study of the subject.
Huxley, T. H.The Crayfish: an Introduction to the Study of Zoology. International Science Series, vol. xxviii. London, 1880.Stebbing, T. R. R.A History of Crustacea: Recent Malacostraca. International Science Series, vol. lxxiv. London, 1893.Calman, W. T.Crustacea. A Treatise on Zoology, edited by Sir Ray Lankester. Part vii., fascicle iii. London, 1909.Smith, G., andWeldon, W. F. R.Crustacea. The Cambridge Natural History, vol. iv. London, 1909.Lister, J. J.Crustacea, in "A Student's Textbook of Zoology," by Adam Sedgwick. Vol. iii. London, 1909.
Huxley, T. H.The Crayfish: an Introduction to the Study of Zoology. International Science Series, vol. xxviii. London, 1880.
Stebbing, T. R. R.A History of Crustacea: Recent Malacostraca. International Science Series, vol. lxxiv. London, 1893.
Calman, W. T.Crustacea. A Treatise on Zoology, edited by Sir Ray Lankester. Part vii., fascicle iii. London, 1909.
Smith, G., andWeldon, W. F. R.Crustacea. The Cambridge Natural History, vol. iv. London, 1909.
Lister, J. J.Crustacea, in "A Student's Textbook of Zoology," by Adam Sedgwick. Vol. iii. London, 1909.
Baird, W.The Natural History of the British Entomostraca. (Ray Society.) London, 1850.Bell, T.A History of the British Stalk-eyed Crustacea. London, 1853.Spence Bate, C., andWestwood, J. O.A History of the British Sessile-eyed Crustacea. 2 vols. London, 1863 and 1868.Brady, G. S.A Monograph of the Free and Semiparasitic Copepoda of the British Islands. (Ray Society.) 3 vols. London, 1878-1880.
Baird, W.The Natural History of the British Entomostraca. (Ray Society.) London, 1850.
Bell, T.A History of the British Stalk-eyed Crustacea. London, 1853.
Spence Bate, C., andWestwood, J. O.A History of the British Sessile-eyed Crustacea. 2 vols. London, 1863 and 1868.
Brady, G. S.A Monograph of the Free and Semiparasitic Copepoda of the British Islands. (Ray Society.) 3 vols. London, 1878-1880.
These works, although still valuable, and indeed indispensable, are now more or less out of date. A list of British Malacostraca (except Amphipoda) will be found in Mr. Stebbing's volume mentioned above.
Sars, G. O.An Account of the Crustacea of Norway. Vol. i., Amphipoda, 1890-1895. Vol. ii., Isopoda, 1896-1899. Vol. iii., Cumacea, 1899-1900. Vols. iv. and v., Copepoda, 1903 (in progress). Christiana and Bergen.
Sars, G. O.An Account of the Crustacea of Norway. Vol. i., Amphipoda, 1890-1895. Vol. ii., Isopoda, 1896-1899. Vol. iii., Cumacea, 1899-1900. Vols. iv. and v., Copepoda, 1903 (in progress). Christiana and Bergen.
A very large proportion of the British species in the groups mentioned are described and figured in this splendid series of volumes. The text is in English.
Norman, A. M., andScott, T.The Crustacea of Devon and Cornwall. London, 1906.Webb, W. M., andSillem, C.The British Woodlice. London, 1906.Memoirs of the Liverpool Marine Biology Committee, edited by ProfessorW. A. Herdman. A useful series of monographson the structure and life-history of common British marine animals and plants. The following relate to Crustacea:Memoir VI. Lepeophtheirus and Lernæa (Parasitic Copepoda). ByA. Scott. 1901.Memoir XII. Gammarus (Amphipod). ByM. Cussans. 1904.Memoir XIV. Ligia (Isopod). ByC. G. Hewitt. 1907.Memoir XVI. Cancer (Edible Crab). ByJ. Pearson. 1908.Descriptions of all the British species of Barnacles will be found inDarwin's"Monograph of the Sub-class Cirripedia." (Ray Society.) 2 vols. London, 1851-1854.
Norman, A. M., andScott, T.The Crustacea of Devon and Cornwall. London, 1906.
Webb, W. M., andSillem, C.The British Woodlice. London, 1906.
Memoirs of the Liverpool Marine Biology Committee, edited by ProfessorW. A. Herdman. A useful series of monographson the structure and life-history of common British marine animals and plants. The following relate to Crustacea:
Memoir VI. Lepeophtheirus and Lernæa (Parasitic Copepoda). ByA. Scott. 1901.
Memoir XII. Gammarus (Amphipod). ByM. Cussans. 1904.
Memoir XIV. Ligia (Isopod). ByC. G. Hewitt. 1907.
Memoir XVI. Cancer (Edible Crab). ByJ. Pearson. 1908.
Descriptions of all the British species of Barnacles will be found inDarwin's"Monograph of the Sub-class Cirripedia." (Ray Society.) 2 vols. London, 1851-1854.