26 



THE POPULAE EDUCATOR. 



The land crabs (Gecarcinus*) present us with the remarkable 

 fact of terrestrial animals breathing by gills. Most fish die if 

 out of the water for a few minutes, but a long submersion in 

 the sea would actually kill these crabs. The gecarcinians visit 

 the shore once a year only, for the purpose of depositing their 

 eggs in the water. During the rest of the year these animals 

 live in moist holes, in woods and rocky crevices, hiding in the 

 day, and coming out to feed at night. The land crabs have 

 a peculiar apparatus near the gills for holding water, so that 

 the branchiae are always kept moist. The species of gecar- 

 cinians inhabiting the South Sea Islands, and feeding upon 

 cocoa-nuts, are said to visit the sea-shore every night to dip 

 their gills in the water. 



The. mountain crab of Jamaica is declared by epicures to 

 make a most delicious stew when caught at the proper time 

 and seasoned with lime juice. 



We must now make a few remarks on the general structure 

 and physiology of these Crustacea. Though crabs are classed with 

 decapods, they have, in strictness, but eight feet, the front pair 

 of limbs being mandibles, which serve as hands, and are not 

 intended to aid progression. The well-known sideway motion 

 of crabs is a necessary result of the peculiar manner in which 

 the joints of the legs are hinged one to the other. Lost limbs 

 are usually restored at moulting time, when the crab throws off 

 its old shell. After a claw has been torn away, the blood ves- 

 sels and nerves shrink at the point of separation, and from the 

 hollow thus formed the new limb afterwards grows. A crab 

 which had lost seven legs in battle, recovered them all about 

 three months after, when the shell was cast. The new limbs 

 were perfect in all respects, but somewhat smaller than the 

 former set. 



These repeated castings of so hard a shell, and the extrica- 

 tion of all the complex limbs from their stony covering, are not 

 the least remarkable phenomena in a crab's life. When the 

 animal ceases to increase, these changes doubtless cease; but 

 during growth several such unclothings must occur. Crabs 

 have been taken covered with oysters of seven years' growth, 

 a clear proof that during the whole of that period the shell 

 had not been cast. These crabs had, of course, ceased grow- 

 ing. How a creature of so complex a form withdraws its 

 body from the tightly-fitting shell, leaving the latter, to all ap- 

 pearance, entire, is a puzzling problem. So completely does 

 the rejected covering retain its form that an observer might 

 easily mistake it for another but smaller crab than its late 

 owner. We say smaller, because the animal after moulting is 

 always larger than the coat it has just put off. 



The operation is by no means a pleasant one, if we may 

 judge by the symptoms of decided " illness " shown by the 

 crustacean. It ceases to eat, wriggles about, rubs against the 

 rocks, and acts like a creature " out of sorts." A thin skin is 

 prepared under the shell before this is thrown off, and the under 

 coat soon hardens into a new covering* If we take up one of 

 the shells soon after the moult, we may, with close inspection, 

 generally detect a crack where the crust was split when the 

 body was being drawn out. 



It has been lately ascertained that the crab undergoes a com- 

 plete metamorphosis, all the changes having been traced from 

 the egg to the perfect crab. The first form of the young crus- 

 tacean is utterly unlike the parent, and was formerly a puzzle to 

 zoologists, who, while they named it Zoea, could only guess at 

 its nature. These creatures, after casting their shells, pass into 

 the second state, in which they bear some resemblance to a 

 shrimp. From this condition they at length emerge as per- 

 fect crabs. This metamorphosis has now been proved to occur 

 in no less than seventeen genera of short-tailed decapods. 

 The first hint of these transmutations was given by Slabber, a 

 Dutch naturalist, a hundred years ago; and the truth was 

 gradually proved by Mr. Vaughan Thompson, Mr. Couch, and 

 M. Milne-Edwards. 



The crab, of course, breathes by gills or branchia3,f as they 

 are often called. These occupy two cavities in the chest, the 

 water being admitted to them through a slit in the side of the 

 thorax, and ejected by an opening near the mouth. The 

 animal is able to regulate the outflow of water from the gills 

 by a movable plate which, turning on a pivot, can be opened or 

 shut at pleasure. By raising the carapace a little, the breath- 



* The Greek for land crab. 



t The Greek for gills. 



ing apparatus, consisting of thousands of fine filaments, may 

 be seen in action. 



The crab's heart is placed just under the carapace, and is 

 simply a strong muscular pouch or ventricle, from which the 

 blood is sent over the body, and also to the gills, resembling in 

 this respect the action of the human heart. The blood is mostly 

 colourless, though sometimes a bluish tint may be noticed. 



The nervous system of the crab consists of numerous ganglia 

 (nervous centres) placed in the abdomen and chest, from which 

 the fine nerve-threads radiate through the body. The most im- 

 portant of these nerves are beautifully exhibited in the hermit 

 crabs dissected by Professor Owen, and now to be seen in. the 

 museum of the College of Surgeons. 



The eyes are compound ; each consisting of numerous six- 

 sided tubes, every one of which forms a distinct eye or eyelet. 

 These organs being placed on short tube-like bodies, retractile 

 in some species, all such crustaceans are called " stalk-eyed." 



The crab's ears are supposed to be concealed under a plate, 

 on the lower part of the second antenna. If this plate be re- 

 moved, a fine membrane will be seen, covering a cavily filled 

 with a fluid, in wh'ch the nerve of hearing is visible. 



The carapace, Ox jhelly covering of the crab, corresponds to 

 the epidermis, or outer skin, in man, but forms, in reality, the 

 skeleton of the Crustacea. It generally consists of about 

 twenty-one flat shelly rings, and is formed of carbonate and 

 phosphate of lime. The peculiar colom-ing matter is produced 

 by the inner skin, or corium, and becomes, under the action of 

 boiling water, of a reddish hue. The lime, phosphorus, carbon, 

 and other elements forming the shell, are secreted by the 

 animals from the sea water. 



The crab has found a place in heraldry, several old families 

 having adopted this crustacean among their insignia ; and 

 even that warlike and chivalrous king, Francis I. of France, 

 did on one occasion, at least, introduce the animal into his 

 shield. This, however, is said to have been a satirical allusion 

 to the hesitating movements of the English army in France, 

 under the Duke of Suffolk, in 1523. 



All crabs discharge a most useful duty, being in fact the 

 scavengers of the sea, consuming the dead animal matter which 

 would otherwise do much to corrupt the waters along the coasts. 

 Thus even the voracity of the crab promotes the well-being of 

 the natural world, and consequently benefits the human race. 



LESSONS IN GEOGRAPHY. XXVII. 



WE now resume and continue the table of latitudes and longi- 

 tudes of places in the Continent of Europe, commenced in our 

 last lesson (see page 389, Vol. II.). 



TABLE OF LATITUDES AND LONGITUDES OF PLACES IN 

 EUEOPE. 



