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 Tria.s.sic epoch, however, and more abundantly in the succeeding Jura.s.sic, 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 (_aeger_, 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; and _aeger_ agrees 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 Penaeidea, which occupies in many respects a primitive place among the Natantia, is abundantly represented in the Jura.s.sic epoch, especially in the lithographic stone (Upper Jura.s.sic) 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 Jura.s.sic and later fossils are of so modern a type that they have been referred to the existing genus _Penaeus_.

The Upper Jura.s.sic 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 Acanthephyridae, 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 Tria.s.sic 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 (Glyphaeidae) now wholly extinct, and are in many respects more primitive and lobster-like than any of the living Spiny Lobsters and their allies (Palinuridae and Scyllaridae). Forms with greatly thickened antennae, indicating a transition to the Palinuridae, begin to appear in the Jura.s.sic; and in the later Cretaceous a genus, _Podocrates_, occurs which is hardly to be distinguished from _Linuparus_, now living in j.a.panese seas. The Scyllaridae have the antennae modified into broad shovel-like plates, and perhaps take their origin from _Cancrinus_, in the Solenhofen lithographic stone (Jura.s.sic), which has broad and apparently flattened antennae. True Scyllaridae are certainly found in Cretaceous deposits, and some, from the Upper Chalk, are even referred to the existing genus _Scyllarus_.

The Anomura are almost unknown as fossils, but the true Crabs, or Brachyura, are abundantly represented. They first appear about the middle of the Jura.s.sic epoch, and, as already pointed out, the earliest forms (Prosoponidae) are referred to the Dromiacea, and appear to be closely related to the primitive h.o.m.olodromiidae now living in the deep sea (p. 134). One of the oldest, and at the same time one of the most completely known, is _Protocarcinus_, 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 the Cretaceous 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 as _Cancer_, 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.

APPENDIX

I. METHODS OF COLLECTING AND PRESERVING CRUSTACEA

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 coa.r.s.e muslin to a stout wire ring of a few inches diameter, is useful for fishing in rock pools. Sh.o.r.e-collecting in this manner is most productive at spring-tides, when the deeper levels of the sh.o.r.e 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, c.u.macea, 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 dish of 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 are to 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 gla.s.s 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 successful for 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 pa.s.sage.

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 quant.i.ties 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 c.u.macea, 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 gla.s.s 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.

II. NOTES ON BOOKS

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 the _Zoological 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.

GENERAL WORK

_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._, and _Weldon, 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.

BRITISH CRUSTACEA

_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._, and _Westwood, 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., c.u.macea, 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._, and _Scott, T._ The Crustacea of Devon and Cornwall. London, 1906.

_Webb, W. M._, and _Sillem, C._ The British Woodlice. London, 1906.

Memoirs of the Liverpool Marine Biology Committee, edited by Professor _W. A. Herdman_. A useful series of monographs on the structure and life-history of common British marine animals and plants. The following relate to Crustacea: