Part 14 (1/2)

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 soe show hints of an affinity with the Decapods In the Triassic epoch, however, andJurassic, a number of types are found which seem to include priroups of Natantia and Reptantia It is noteworthy that a 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 rees with theh there is little else, in what is known of its structure, to help to determine its affinities

The tribe Penaeidea, which occupies inthe Natantia, is abundantly represented in the Jurassic epoch, especially in the lithographic stone (Upper Jurassic) of Solenhofen, and by soree 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 enus _Penaeus_

The Upper Jurassic rocks also preserve the earliest undoubted specimens of true Prawns of the tribe Caridea, and so branches (exopodites) on the thoracic legs, so that they were probably related to the pri 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, fore 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 (Glyphaeidae) noholly extinct, and are in many respectsSpiny Lobsters and their allies (Palinuridae and Scyllaridae) For a transition to the Palinuridae, begin to appear in the Jurassic; and in the later Cretaceous a genus, _Podocrates_, occurs which is hardly to be distinguished fro in japanese seas The Scyllaridae have the antennae modified into broad shovel-like plates, and perhaps take their origin froraphic stone (Jurassic), which has broad and apparently flattened antennae True Scyllaridae are certainly found in Cretaceous deposits, and soenus _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 Jurassic epoch, and, as already pointed out, the earliest forms (Prosoponidae) are referred to the Dromiacea, and appear to be closely related to the pri 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 Wiltshi+re, which is preserved (in the only known speci that the abdomen was less closely doubled under the body than in roup 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 becoenera, such as _Cancer_, date back to the Eocene The Spider Crabs (Oxyrhyncha) are rare as fossils, and the earliest speci of the Tertiary

APPENDIX

I METHODS OF COLLECTING AND PRESERVING CRUSTACEA

It ive here a few hints as to theCrustacea Of the species that live in the sea,over stones and searching a sea-weeds and in rock crevices A s of coarseof a few inches dia in rock pools Shore-collecting in this -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 loater e 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 athe operation two or three ti are washed out into a jar or dish of sea-water for exa is theCrustacea that live in deeper water The dredge usually eular iron fraer sides of the fraed and bevelled outwards, so as to ”bite” when the dredge is dragged along the sea-botto in rings The dredge-rope is s, while the other is held only by a ”stopping” of yarn, which gives way if the dredge should catch on a rock, and pered sideways off the obstruction The size and weight of the dredgeto the depth at which it is to be used and the power available for working it A convenient size for use with a s boat at moderate depths has a fra, many speci 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 speci the operations for the repair of sub animals of the surface of the sea are to be captured byof -cloth,” attached to a galvanized iron-ring of one or two feet in dialass 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 froanisms 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-er Crustacea, even though these h which it is drawn

By weighting the tow-net itanimals of mid-water When it is so used, however, it is impossible to tell from what depth any particular speci the hauling in of the net

Fortow-nets” of various types have been devised, which can be opened by a ”er”

sent down the line when the net has reached the desired depth, and closed again by another ”er” before it is hauled in

A si s on a sandy bottoht 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 s the smaller fresh-water Crustacea--Water-fleas and the like--a s-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 Shris and wells In wells fitted with a pump, Professor Chilton found that ”the Crustacea are often brought upis first co the handle of the pu at the ordinary rate” In disused open wells, they -net with a piece of staleit 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 fore has been devised with runners to prevent it fro into the soft mud, while the mouth of the net is raised a few inches above the botto Crustacea the bestspirit diluted with a little less than one-third its bulk of water gives about the required strength If too strong spirit is used, the specianisms become shrivelled Methylated spirit as sold in the shops in this country contains mineral naphtha, and turns milky ater is added, so that it is unsuitable for preserving speciht, 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 ile, and s much lime, such as cumacea, may become decalcified For Crustacea collected by the tow-net, however, for 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 th of foranisms, but 5 per cent

(_ie_, 1 part of coenerally useful

Crabs, Prawns, and the like, if put alive into strong spirit, id that these break on the slightestthe ani in strong spirit Marine speciestaken not to allow theer than is necessary, as it causes distortion of the er Crabs, Lobsters, and the like, h in this state they are unsuitable for exaes The carapace should be detached, and the soft parts cleaned away as far as possible, a bent wire being used, if necessary, to res 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 specilass 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