AVERY beautiful inhabitant of small ponds is the Fairy Shrimp, which much more closely approximates to our idea of a typical crustacean than any of the preceding examples. It is transparent, with a reddish forked tail and reddish limbs. The whole body is ringed, and the first eleven rings (exclusive of the head) carry paddle-like legs. The eyes are large and black and are elevated on stalks. Fairy shrimps have the curious habit of lying on their backs and drawing a current of water and food particles to their mouths by paddling with the feet. The small ponds in which they live may often dry up, and in this case their eggs lie dormant in the mud at the bottom, ready to develop when moisture comes again. The egg gives rise first to a nauplius larva which, by a series of moults, gradually assumes the adult form, acquiring segments and limbs.
We must now turn to that section of the Crustacea which includes the crabs and crab-like animals, such as the lobsters, shrimps and prawns. Other members of this section are the sand hoppers, which are often seen in myriads on sandy shores, and the familiar wood lice of our gardens. In the forms we have so far studied, there has been no definite division of the body into well-marked regions, but now we can easily identify three regions : the head which bears the eyes and antenna?; the thorax with eight pairs of walking appendages, some of which may be modified for special purposes; and the abdomen, consisting of six rings, each bearing one pair of appendages. The abdomen terminates in a tailpiece without appendages. The structure of the thorax is often hidden by a protective horny cover-shell, or carapace, which sometimes fuses with the armour of the head, as in the crayfish.
In the case of the shrimps, lobsters and crabs, the first three pairs of thoracic legs are unlike the remaining five pairs which are used for walking (the name of the order describes these five pairs of legs, I.e. Decapoda or ten feet) and are modified to act as foot-jaws. Of the five pairs of walking appendages, one or more have pincer-like extremities and are used for food-capture or defence; these are very obvious in the crabs. The gills are feathery structures at the bases of the legs; the blood passes through the filaments of the gill and exchanges its carbon dioxide for some of the oxygen dissolved in the
water. By placing a little powdered dye near the last walking legs, one can perform a simple experiment on the crayfish which will show the way in which a current of water is created by the paddle-action of the mouth-parts. The water passes under the shell near the last pair of legs and out again near the head in such a way that a constantly changing stream of water passes over the gills so that the animal’s oxygen supply is continually being replenished.
The tendency in this order (Decapoda) is towards the reduction is size of the abdomen. In the crabs, the abdomen is very small and is tucked under the thorax; in the shrimps, however, it is quite large and its swimming limbs are the principal loco-mo tory organs. These changes are connected with the walking habits of the crabs and the swimming habits of the shrimps. In view of these characteristics, we could speak of the crabs as the Short-tailed Decapods and the lobsters and shrimps as the Long-tailed Decapods. The Hermit crabs come into a different category, as their tails are asymmetrical. The crab-like animals can therefore be split up into three sub-orders :
THE MACRURA or long-tailed forms like lobsters, with strong abdominal limbs. THE ANOMURA or irregular tailed forms like the hermit crabs, with a twisted abdomen. THE BRACHYURA or short-tailed forms like the true crabs with small abdomens and few abdominal
The long-tailed Macrura include swimming and walking forms like the prawns, shrimps and lobsters. Prawns and shrimps are frequently seen in the rock-pools left by the receding tide; the shrimps lie on the sandy bottom of the pool, half-covered in the sand, and it is only when the water is disturbed that they can be seen darting about in search of a new retreat. Many of the prawns possess the power of colour change.
The lobsters lurk in crevices in the rocks; their habits can be studied in the aquarium at a zoo. They back into these little caves, and often only the large, pincer-like limbs can be seen, waiting to seize any suitable prey. The large pincer claws differ from one another in size and shape, one pair of pincers being large and knobbed, the other smaller and sawlike. The large one is used for breaking up shell-fish, the other for clasping and tearing the prey. If a lobster is caught by one
of its legs, it has little compunction about leaving its leg in your fingers and making its escape. This is not so drastic a measure as one would think, for its legs are provided with special breaking points for this purpose and, what is more, it can grow another leg to take the place of that which was lost—in other words, the limbs can be regenerated. The eggs of the lobster give rise to little transparent larvae which swim near the surface of the sea—this in contrast to their parents’ habit, which is to remain on the bottom.
The Crayfishes are fresh-water relatives of the lobsters, and they live in some of the English and Irish rivers; they burrow holes in the banks and either await their prey there or go hunting along the bottom. When alarmed they retreat with great rapidity by means of strong strokes of their tail fans and
contraction of the abdominal muscles. The young crayfishes are like their parents and spend their early days in clinging to the swimmerets of the mother’s abdomen. The green crayfish of England is seldom used as food, the red continental form being specially imported for this purpose.