A COMMON CRAB.

Otago Witness, Issue 2785, 31 July 1907, Page 81

A COMMON CRAB.

By Dixomns.

As the subject of a little dissertation upon evolution, the common chore-crab n?ay serve as well as any other animal — better, indeed, than many animals higher in the scale of being, in that, extraordinary as the facts of crab development axe, they confined within a relatively narrow compass. The individual development of « mammal, bird, reptile, or fish is an exceedingly complex affair, and the rapidity with which it proceeds is remarkable. These important groups comprise all the vertebrate or back-boned animals, but the crab is _a typical high-class arthropod, or segmented animal, with legs and various other appendages. The sub-kingdom Annulosa, of which the arfchropoda constitute the most important division, is an enormously populous one, thronged as it is by an aristocracy of crustaceans, spiders, scorpions, centipedes, and insects; and having a proletariat of exceeding great variety in shape of spoon-worms, leeches, earthworms, tube-worms, sand-worms, and arrow-worms. Needless to say, this population has its representatives wherever life is to be found, on land, in the sea, and) ill all fresh water everywhere.

! ' » Zoea, or larva of the crab. "When first observed by naturalists it was described as a new species of crustacean, and its true relationship was fox long unsuspected. .In the waters of our beautiful harbour of Otago quite a number of interesting members of the great crustacean claes are to be found, some of high degree and some of low. On the broad sballovrs, at tho very head of the harbour, there are present vast numbers of a kind, of crab whioh burrows in the none too fragrant mud of these expansive flats. Very knowing and very cunning these cave-making shellbacks look as they straddle about near their holes, looking for such trifles of garbage as luck, or the receding tide, may have left within reach. A thump with one's foot on tfce easily-disturbed surface, and they disappear beneath it with remarkable celerity. Leaving them there, and following the Portobello road until clearer water is reached, we will be in the way 1 of finding other shaped and more cleanly-

living crustaceans. Th«ee, are greenish- •' lined, semi-translucent,-, crayfish-like ani-i mals, which find their caves amongst tbs stones of the embankmenriv They aip two or three inches in length,-^and.ivery : elegan,t j they appear as they dark oufcy (tail.ifirst/* for a little distance, retrjea^ing: £$ gxa<?ofully and rapidly on. the,-, leasi*.- appearance of anything suspicious witthin, range. These, I dareeay, are prawns, of some.kiad., Then there are the "whaierfeed" — pretty, lohsterlike creatures, often tinging th/ - t water- 'ae-, with great splotches of- Wood, -in- numbers so numerous ,40* they s j»m|ho : nly i .a^B^arp, When, as frequently hWgßens,' v Jtnillion6 of these small crustaceans' gek***left** along the beach, their beauty ceases to be appaneat, and an execrable odow poßufcesr , the freak bay breezes..: „.ji \ y .... .. „*;^n-.,> There are -other ci-ab-it6latives> ia^plentyeverywhere • in/ oar ' .nroghboar&c&fih Jl Tfcece are "elafcert" t>ft'lc&d aaid samdiopjiers bn ' the sliere noV'tpi l speak of the numerous ' species, mhabitiflg fttefdoean, including trie 1 misnamed "cfi^fisW^ -eaten by people- who;*' have scant regard 'lor the terrors of ihda- - gestion.' In almost every" freshwater creek ' the true crayfek is t& Ver^oMd, and airpond waters swarm *n^''mstii6et cntsfcaceous orgakffiin^ of J ma;hy' i lqMafe. raHf, from the m^bera'"'of i tiM : gfei)fc )< tiMsT(m t there is literally no* gettirig'aw4^r > .aiid''tlii6 shore-crabs which swarm jn myftacfe among the shingle ail up Tnod-'So-tilt ±h& i'-fiiiW atre probably the most' kfeVfttasl^. visible" of all tie family. I taye;j&t^, J^/'focil o because it is common, but' beeiuise it 1 ex- . hibits, along with all Mie 'criity^ ."^ furthest stage of evolution attained by crusted animals. The lobster and £ke fish are highly -developed species, feutti^y, are distinctly inferior to tie crab. , 4- Uglance at tbe lower figures of the three , which illustrate my remarks will render this plain. It will be seen that the lobster form represents a stage only in the full development of the crab, n

The young crab in its megalopa stage. ' Development is well advanced, but it etill retains the lobster-like ''tail." 1 As described by naturalists who have made a careful study of its life-history, the stages of crab development are somewhat as follows : — When, the baby crab, or zoea, leaves the -egg it k extremely unlike its parents ; i its portrait, as given above, will convince anyone of that fact. As a baby, it looks like a big head with various fantastic appendages attached thereto. Such a body, or tail, as it possesses is as unarablik© as well oould be. It has one huge spike sticking up on top of its head and another pointing downward like a beak. It owns a pair of good, serviceable eyes, but they are not arranged on the crab plan of optical construction, having no "stalks." The larval crab, or zoea, is a free-swimming creature, and moves through the wtater hy aid of two out of three pairs of wihai may be called legs, only they are useless for walking with. The front pair ultimately become "feelers," or antennas, and the other two pairs, from being paddles, .become what are usually looked upon as the crab's jaws, but are in reality "jaw-feet," organs whose me consists in dragging the food) of the animal into ita gullet. The plan of crabstructure differs widely from that of the vertebrates, and the mouth parts show practically no correspondence. As a mat' ter of fact, the crab has its teeth in its stomach, and if we had a similar arrangement the dentist would require to turn us inside out should a "filling" or an extrac- | tion or a "new set" be needed. I Like other animals, impelled by forces over which it fees no control, the larval i crab grows. As it grows it occasionally • ! moults its skin, and, though not -exactly a lightning-change artist, still' -every motoit' shows it a stage nearer to 4he ideal perfection, of crab form. Wlsen' it- has got to what is known as the 'm^gaiopa stage it is to all intents aad purposes ft cxnb with a lobster's' tafl. Its limbs are of the right shape and number, and the serviceable "nippers" ate wdl in evidence. The eyes no longer look like-' the lens of a bull's-eye lantern, trai are placed on "stalks," by the free movement of which the youthful crab is eomodern-biy aided in ite efforts ±a> see the world, acel to maintain a wary outlook upon its immediate neighbours. Phe lobster-like tail gradually disappears to the extent of becoming the mere vestigial affair known as the crab's "apron." The crab is now arrived at its full development, practically all head and chest, the "apron" being all that is left of the abdominal segments so characteristic of the lobster and crayfish. Those who have observed the test -named animals in a state of^nature will remember how in each kind the eggs are carried by the females attached to the under side of the tail segments. In this regard the crai> also continues to resemble its less-specialised relations — it carries its rich mass of "berry," or ova, beneath its vestigial tail until sucJi time as the eggs hatch out and set forward; in the line of development roughly sketched above. The differences that separate the higher Crustacea from man and the rest of the primates are wide indeed ; in the one case the skeleton is external and invertebrate, in the other It is internal and vertebrate; But it is worth noting that in bath groups* high development is intimately associated X

iwith .the, gradua,Land practically complete v elimination of the tail!