The Life of Crustacea

by W. T. Calman

The eyes, as already mentioned, are set on movable stalks, so that they can be turned in any direction at the will of the animal, and are of the type known as "compound eyes." If the convex black area at the end of the eye-stalk be examined with a strong lens, it will be seen that the membrane which covers it is divided up into a beautifully regular series of square facets. This membrane is a thin and transparent continuation of the chitinous covering of the body, and if it be stripped off and examined under a microscope, it will be found that each facet is capable of acting as a lens and forming an image of external objects. It is not to be supposed, however, that the Lobster sees a separate image in each of the facets, some thirteen thousand in number, which go to make up each eye. In the interior of the eye, at some distance from the surface, are a large number of rod-like bodies, connected with the fibres of the optic nerve, and believed to be the actual organs for the perception of light. Each rod corresponds to one of the facets, and as it lies at the bottom of a long conical tube, of which the walls are covered with dark pigment, it can only receive light from a single point in line with the axis of the tube. In this way the image of any object will be built up, like a mosaic, out of the impressions of light and darkness received through the separate facets, and transmitted to the underlying rods. It has been shown in some Crustacea that, when the animal is in a very dim light, the curtain of pigment separating the tubes is partially withdrawn, so that the light from each facet can reach, not one, but several rods. In this way the images of objects received are much brighter, although they are less sharply defined.

It might be thought that in animals like the Lobster, enclosed in a hard shelly covering, the sense of touch must be very dull, if not altogether absent. This, however, is not the case. What is probably a very delicate tactile sense is provided for by the numerous hairs which are found, of many sorts and sizes, all over the body and limbs. Each of these hairs is really a hollow outgrowth of the chitinous covering, containing a delicate prolongation of the soft tissues underneath, and also supplied, in many if not in all cases, with a nerve-fibre, so that the slightest movement of the hair caused by contact with a solid body is perceived by the animal. Many of these hairs are themselves beset with delicate secondary hairs, arranged so that the whole looks like a feather or like a bottle-brush. These hairs are adapted for detecting slight movements or vibrations in the surrounding water.

Whether Crustacea living in water can hear, in the sense in which the word is used of animals living in air, is doubtful; but it is certain that they are extremely sensitive to vibrations only a little coarser, so to speak, than those we know as sound. The Lobster, and many other Crustacea, do indeed possess a structure which was long supposed to be an organ of hearing, and may possibly in part fulfil that function, although it is now known that that is not its only or even its chief use. It consists of a small cavity in the basal segment of the stalk of the antennule, opening to the outside by a narrow slit on the upper surface of the segment. The cavity is lined by a delicate continuation of the chitinous covering of the body, and has on its inner surface a series of feathered hairs of the kind described above, which are richly supplied with nerve-fibres from a large nerve entering the base of the antennule. Within the cavity, and for the most part entangled among these hairs, are a number of grains of sand. When the Lobster moults, the lining membrane of this cavity is thrown off like the rest of the exoskeleton, and with it the contained sand-grains. While the shell is still soft after moulting, and the lips of the slit are not rigid, as they afterwards become, fresh sand-grains find their way into the cavity to take the place of those which have been cast off. Perhaps, like some other Crustacea, the Lobster buries its head in the sand to insure that some grains may find their way in; for its pincers are too clumsy for it to pick up sand-grains and to place them in the cavity, as some Prawns have been seen to do. At all events, if a freshly moulted Prawn be placed in a vessel of sea-water, and supplied, instead of sand, with powdered glass or metal filings, particles of glass or metal will after a short time be found in its antennular cavities. This habit has been utilized in a very ingenious experiment by which the function of these organs was demonstrated. A Prawn had been induced in this way to place particles of iron filings in the cavities, and a strong electro-magnet was brought near the side of the vessel in which it was kept. It was observed that the Prawn, which had been swimming in the usual horizontal position, at once turned the under-side of its body towards the magnet, and swam about on its side as long as the magnet was in action. When the current exciting the magnet was cut off, the animal resumed its ordinary position. This experiment shows that these organs, to which we may now give their proper name of statocysts, are organs for perceiving the direction of the force of gravity. The magnetic force acted on the particles of iron in the same way that the force of gravity acts on the sand-grains in normal conditions, and the Prawn felt the weight of them, so to speak, pulling towards the side instead of the bottom of the vessel, and turned its body accordingly, to swim, as it supposed, right side up. It is now known that those parts of the human ear called the "semicircular canals" have a somewhat similar function as "organs of orientation," although to animals walking on the solid ground this function is not so important as it doubtless is to animals swimming in water.

The sense of smell is believed to have its seat chiefly in the antennules. The outer branch of each antennule bears tufts of peculiar hairs, in which the chitinous covering is extremely delicate, so that substances dissolved in the water can easily pass through and affect the nerve-endings within. These hairs are known as "olfactory filaments."

The sense of taste in aquatic animals is, perhaps, not sharply defined from that of smell, but it is not very rash to assume that certain hairs on the mouth parts and on the fleshy upper and lower lips which bound the opening of the mouth have to do specially with this sense.

The relative importance of the various senses in the Lobster is well illustrated in the following account of its habits given by Dr. H. C. Williamson in the Report of the Scottish Fishery Board for 1904. After noticing that, in daylight at least, the Lobster appears to be "purblind," only distinguishing light from shadow, Dr. Williamson goes on: "It tests a shadow with its antennæ, or sometimes, when a strong shadow is thrown on it, it jumps at it with its chelæ outstretched and snapping. It is dependent on its antennæ for guiding it in safe places. It is especially careful in testing any hole before it is satisfied with it. It discovers the cavity by means of its antenna, which is waved well out to the side and in front as it walks. It searches the innermost depths of the hole with the antenna, and then inserts its chela. If the examination with the chela is also satisfactory, it immediately turns and backs smartly into the hole. In feeding it is guided to the food by the antennules. A piece of food which is dropped near a Lobster may fall quite unnoticed unless it happens to touch the antenna or the [legs]. It is not seen at all. But sooner or later, according as the distance is short or great, the scent of the food, carried by the currents set up by the exopodites of the maxillipeds, reaches the Lobster. The Lobster is immediately excited, although previously it was lying quite inert in its hole. It whips the water with its antennules in a staccato fashion, and feels about with the antennæ and chelæ, at first without leaving its hole. At once both antennules are seen to be whipping in the direction in which the food is lying, and an active search is made with the antennæ. If they do not succeed in locating the bait, the Lobster rather reluctantly leaves its hole, but cautiously, feeling all round about with its antennæ. It goes off straight in the direction in which the food is lying, and, if it misses it with its antennæ and chelæ, walks over it and gets it with its chelate [walking legs]; it usually picks up its food with the second [walking leg]. Meanwhile the expected feast has by association stimulated the maxillipeds, which are actively working as if they were already masticating the food. Once the food is seized it is conveyed to the maxillipeds, and the Lobster retreats to its hole, there to enjoy its meal."

Lobsters, like most other Crustacea, are of separate sexes. The females (see [Plate I].) may be distinguished from the males by the fact that the abdomen is broader and has deeper side-plates, and by differences in the form of the first two pairs of swimmerets. In the female the first pair, which have only one branch, are short and slender filaments, while in the male they are stout and peculiarly twisted rods. The second pair in the female are similar in form to the succeeding pairs, but in the male they have an additional lobe on the inner branch. The openings of the generative organs will be found in the male on the basal segments of the last pair of legs, while in the female they occupy the same position on the legs of the last pair but two. The testis of the male lies in the thorax, just below the heart. The ovary, which has the same position in the female, is usually much more conspicuous, and from its red colour in the cooked Lobster it is known as the "coral." On the under-side of the thorax of the female, between the last two pairs of legs, is a three-lobed structure enclosing a cavity known as the "sperm-receptacle." Its function is to receive the fertilizing substance from the male, and to retain it until the eggs are ready to be deposited.

PLATE I