Through a pocket lens

by Henry Scherren

The Prawn is a capital inmate of the aquarium, and as it does well in confinement, specimens should be kept in order to get a general acquaintance with their form and external anatomy, and to watch their habits. The Common Prawn will answer the purpose, but still better is Palaemone´tes varians, an exceedingly common species. It has this advantage, that it ‘seems to be equally at home in salt water and fresh.’ The only condition necessary is a good supply of food, and this may be furnished by putting into the aquarium from time to time a quantity of water-fleas. If these Prawns are well fed they will shed their skins at frequent intervals, and this operation will give us material for examination, for the cast skin will serve our purpose almost as well as a spirit specimen.

Some of these Prawns are now living in one of my aquaria. They were taken in a brackish dyke or cut near Newhaven, in Sussex, and in the mud which was brought back with them were a number of small bivalves of the genus Sphaerium. Most people know Mr. Kew’s exceedingly interesting book, The Dispersal of Shells[44]. In it he relates some extraordinary instances of the way in which species of shells are carried short distances, and may be carried from one district or country to another. These Prawns offered a good illustration of this, and practically confirmed some of the statements in his book, for on several occasions they were seen with the bivalve shells attached to their walking legs. The molluscs lay half buried in the mud and vegetable débris at the bottom of the tank, and as the Prawns walked about they sometimes trod between the open valves, which, as they closed, fastened on to the intruding limb. On one occasion the molluscs did not relax their grasp for days; and had this incident occurred when the creatures were at liberty the molluscs might have been carried for a considerable distance. If specimens of Sphaerium are put into an aquarium containing Prawns of this kind, it is probable that before very long the crustaceans will have one or two attached to some of their limbs.

Prawns are exceedingly beautiful, and if we get hold of live specimens, from salt water or fresh, they should be put into an aquarium—the smaller, in reason, the better—so that their motions may be watched with the hand lens. If much weed be put in, the Prawns will use their walking legs in preference, while if there is little vegetation the powerful tail-fan will be employed for motion backwards, while the five pairs of limbs on the abdomen enable their owners to move forwards through the water.

From Fig. 66 one may get a good notion of a Prawn, and of the points in which Prawns, in the zoological sense of the word, differ from Shrimps. The head of the Prawn is armed in front with a long blade-like beak, studded along its upper and lower edges with a series of teeth like those of a saw, and the second leg is chelate, that is, armed with pincers, resembling, in miniature, that of a lobster or crab. In the Shrimp, on the contrary, there is scarcely a trace of the beak, and the first leg is incompletely chelate, or sub-chelate[45] (Fig. 67), its last joint folding back upon the one that supports it, just as the blade of a pocket-knife closes on its handle. These two distinctions hold good between all Prawns and all true Shrimps.

Fig. 66.—Prawn.

Now let us go over our Prawn—a spirit specimen—in detail. The antennae may be separated, and examined, and the appendages of the inner pair distinguished, for at first it may be thought that there are more than two pairs. This, however, is not the case, as should be ascertained by actual investigation. A needle inserted at the base of the outer antennae will separate the first three segments, bearing respectively the eyes, and the first and second pairs of antennae. The eye should be carefully looked at to make out that it is really compound. Then the joints of the antennae, each with its circle of sense-hairs, are to be noticed. Last of all, the inner pair of antennae deserve attention, for these carry in the basal joint an organ of hearing. This joint is large and sac-like, and contains an opening through which grains of sand are introduced by the animal itself. The grains serve to transmit the vibrations of the water in the sac to the auditory hairs, to each of which a branch is sent off from the auditory nerve. If the joint is opened the sand will be found. The first antennae of a lobster or crayfish may also be examined and compared.

The mouth organs, of which there are six pairs, will offer some difficulty, and for this reason it may be well to pass them over in this case and to deal with these organs generally when treating of the Crab.

Beneath the outer foot-jaws are the first pair of walking feet, which are used as cleansing organs. Gosse describes them as ‘beset with hairs which stand out at right angles to the length of the limb, radiating in all directions like the bristles of a bottle-brush.’ If we watch our Prawn in life, we shall frequently see these limbs in active operation. They are brought to bear on every part of the body within reach. Sowerby says[46]: ‘The prawn loves to be clean, and he takes surprising pains to keep himself so. Drawing up his tail and abdomen, he subjects their under surface to the most careful revision, scrubbing and poking between the lappets of the shell and body, diving into every crevice, and with the pincer-hand picking out every speck too large to brush away.’ The next pair of legs are also chelate; but the three following pairs are armed with claws, and it is upon the points of these that the animal walks on the bottom. The pincers of the second pair of legs are used to pick up food and bring it up to the mouth organs, where it is taken by the outer foot-jaws, and passed into the mouth. The swimming feet carry two branches, finely fringed with hairs.