CHAPTER XV. Functions and Instincts. Crustacean Condylopes. We are now arrived at a Class of animals, in which the organs of locomotion assume a new and more perfect form, corresponding, in some measure, with those of many of the vertebrated animals. The advance in structure, hitherto, from a mouth surrounded by organs like rays, serving various distinct purposes, and by different means contributing to the nutrition, respiration, and motions of the animal, has been, by certain inarticulate organs, more generally distributed over the body, but still in a radiating order; as, for instance, the tentacular suckers of the Stelleridans and Echinidans, which they use in their locomotions, and for prehension, as well as the purposes just named. In the Entomostracans, as we have seen, the legs, though jointed, are very anomalous, assume various forms, and are applied to sundry uses; in the sole instance of the kingcrab, they take the articulations of those of the Crustaceans, in which we may trace the general structure of the legs of the other Classes of Condylopes. But as I shall have occasion, in a subsequent chapter, to give a concentrated account of the gradual development of the organs of locomotion and prehension, from their first rudiments in the lowest grades of the animal kingdom to their state of perfection in the highest, I shall not here, therefore, enlarge further upon the subject, than by observing, that, in most of the Decapod Crustaceans, the anterior legs are become strictly arms, terminating in a kind of didactyle hand, consisting of a large joint, incrassated usually at the base, and furnished on its inner side with a smaller movable one, constituting together a kind of finger and thumb, with which it is enabled to seize firmly and hold strongly any object that its inclinations or fears point out to it. This hand we called the chela or claw, or more properly pincers, of the lobster or crab. We find it also in the scorpion and book-crab, which, on shore, are in some sort analogous to the long-tailed and short-tailed Crustaceans, 1 1 Chelifer. lobsters and crabs of the waters. This structure of the hand, in these creatures, is particularly fitted to their wants and situation. A hand like ours, consisting of a quadruple set of fingers and an opposite thumb, to be of sufficient power for their purposes, must be so disproportioned to their size, as to be an incumbrance rather than a useful instrument of prehension; but as now constructed, it has the requisite strength for the purposes of the animal, without being disproportioned to its size, and inconvenient for its use. Thus we see how nicely every thing is calculated and adjusted by Supreme Wisdom, to the nature and circumstances of every animal form. But these great claws are by no means universal amongst the Crustaceans. In some the claws are very small, but the loss is often made up to these by an increase as to number, so that if they cannot lay hold of large animals, they can seize, at the same time, several small ones. We have seen that in the king-crab all the legs have these prehensory claws, and they vary in number in many of the smaller Crustaceans, as the shrimp, prawn, pandle, &c. The foreleg of some of these has prehensory claws, that are formed like the mandibles or cheliceres of spiders and the arms of the Mantis-whence they are called mantis-crabs. Instead of a forceps, consisting of a finger and thumb, the claw that arms the extremity of the leg is folded down, and received into a channel of the shank, and kept from dislocation by a tooth, or spine, at the base; this structure may be seen in the shrimp. There is another circumstance, distinguishing the decapod, and stomapod Crustaceans, that is peculiar to them, their eyes. are placed upon jointed footstalks, so that when they want to explore and examine what passes around them, they can immediately erect these organs, and so greatly enlarge their sphere of vision, but when they have retired to their retreats in the cavities of the rocks, or to burrows that they have formed, they can place them in repose, in a cavity provided for them by their Creator, in the head. Any person, who casts an eye over these creatures, will be struck by repeated analogical forms, representing some terrestrial animals of the same Sub-kingdom. Thus a large number of those distinguished by the shortness of their tails, the crabs, present, both in their retrogressive and, lateral motions and general aspect, an astonishing resemblance to many Arachnidans; some imitating spiders, and others phalangians: and, 1 Crangon vulgaris. 3 Pandalus. 2 Palamon serratus. amongst the long-tailed tribe the lobsters, one1 very accurately represents a scorpion, and another a mantis." We have seen the same tendency in the Annelidans to approach or imitate terrestrial forms, as if the marine and aquatic animals were anxious to quit their fluid medium, and to become inhabitants of the dry land. The animal living on shore and in the woods at St. Vincent, taken for a Molluscan by Mr. Guilding, appears almost like a creature that had succeeded in such an attempt. All these resemblances and approximations show, that the great Creator embraced at one view all the forms to which he intended to give being, and created no individual without furnishing it with organs which give it some relation to others; or so moulding its outward form, as to cause it to represent some others to which it is clear it is not brought near by any characters, common to both, that indicate affinity. What can more evidently and strongly manifest design, and that of a mind comprehending simultaneously the whole world of created beings, than thus to concatenate all link to link and wheel within wheel, through all their intricate revolutions and ramifications connecting and connected, and all the while reflecting others of a higher or a lower grade with mimic features? this shows the hand, the art, the wisdom, the power, and the goodness of that unfathomable depth and immeasurable height of Deity, which comprehends all things and is comprehended by none; and to whom all things owe their being, and their form, and their organs, and their several places and functions. The general characters of the present class are BODY apterous, covered by a calcareous crust, divided into segments. Legs jointed, 10-16. Mouth composed of a lip. tongue, a pair of mandibles, often bearing a feeler, and two pairs of maxilla, covered by maxillary legs. Spinal chord knotty, terminating anteriorly in a small brain. A heart and vessels for circulation. Respiration by gills. 1 These are divisible into five orders. 1. Decapods. Gills situated under the sides of the shell. Ten thoracic legs. Eyes on a jointed footstalk. 2. Stomapods. Gills attached to five pairs of appendages, or spurious legs, under the abdomen. Eyes as in the Decapods. 3. Læmipods. No abdominal appendages. Eyes sessile. Thalassina Scorpioides. 3 See p. 187. PLATE VIII. FIG. 1. 2 Squilla Mantis. 4. Amphipods. Head distinct. Eyes sessile. 5. Isopods. Head distinct. Eyes sessile. Legs simple, equal. 1. Decapods. This order naturally resolves itself into two sections, viz. The short-tailed Decapods or Crabs,1 which have their abdomen folded under the trunk; and the long-tailed Decapods or Lobsters, Cray-fish, &c., whose abdomen is always extended. Writers on the Crustaceans usually begin with the shorttailed, and then proceed to the long-tailed Decapods, and this arrangement seems natural, when the transit is to those with sessile eyes, such as the locust-crab; but yet, when we consider how nearly related to the spiders the former animals are, and that in the latter, though the head is not formed by a distinct suture dividing it from the thorax, yet its contour is strongly marked out externally by an impression, and internally by a ridge, at least in the lobster and cray-fish,—it seems as if the two tribes should form two parallel lines, and proceed, side by side, towards the Arachnidans and Myriapods. I shall, however, follow the usual plan, and give now some account of the crabs. Of these, none are more remarkable than what have been denominated land-crabs, from their usually living on shore, and making for the sea only at certain seasons. Of the most noted species of these I have already given a full account; but I shall here notice some others, having the same habits, that will interest the reader. Aristotle, long ago, noticed a crab of this description, found in Phoenicia, under the name of the Horseman, which, he says, runs so fast that it is not easy to overtake it." Olivier found this account true of those he saw on the coast of Syria; and Bosc observed a species in Carolina, which he had some trouble to overtake on horseback and shoot with a pistol. These horsemen crabs are found only in warm climates, where they inhabit sandy spots near the shore, or the mouths of rivers. They make burrows in the sand, to which they retreat when alarmed, and in which they pass the night. Another kind of land-crab" is distinguished by the extraordinary disproportion of its claws; one of them, sometimes the left and sometimes the right, being enormously large, while 1 Brachyuri. 3 Orchesia litterea. 5 IANEVS. Gr. 2 Macrouri. 4 See above, p. 66. 6 Hist. Anim. 1. iv. c. 2. 7 Ocypode Hippeus, probably Cancer Cursor. L. 8 Gelasimus vocans. the other is very small, and often concealed, so that the animal appears single-handed. This formation, however, is not without its use-for, when retired into its burrow, it employs this large claw to stop up the mouth of it, which secures it from intrusion, and this organ is in readiness to seize such animals as form its food and come within its reach. They have the habit of holding up the great one, as if they were beckoning some one; but this, doubtless, is an attitude of defence. These crabs live in moist places, near the shore. They attack, in crowds, any carrion, and dispute the possession of it with the vultures: they do not willingly enter the water, except when they lay and hatch their eggs, and it is conjectured that their young are for some time entirely aquatic. One kind of them,1 which forms numerous burrows, remaining in them during three or four months in the winter, usually stops them up, so that the animals are obliged to re-open them when the warmth of the vernal sun bids them come forth again from their winter quarters. They are devoured by numerous animals,-otters, bears, birds, tortoises, and other reptiles, all prey upon them, but their multiplication is so excessive, that there seems no sensible diminution of their numbers. The next tribe of Decapods are the long-tailed ones, which do not fold their abdomen under their body. This part is usually furnished at the extremity with several plates, which the animal expands so as to form a fan of five or six leaves; they are easily seen in the common lobster, like the tail of birds-they are useful to the animal in its passage through an element that requires to be moved by organs of a firmer consistence than feathers. The lateral ones, in the species just named, having a kind of articulation, so that they can be partially depressed, and push against the plane they are moving upon; they do not, like the crabs, quit the water, and are some of them, as the cray-fish, fresh-water animals. I shall begin with a tribe which, in some degree, connects the crab with the lobster-these are what are denominated Hermit-crabs, whose abdomen being naked, and unprotected by any hard crust, their Creator has given them an instinct, which teaches them to compensate this seeming defect, by getting possession of some univalve shell, suited to their size, which becomes their habitation, and which they carry about with them as if they were its proper inhabitants. These crabs 1 G. Pugillator. 3 Astacus fluviatilis. 2 Astacus Gammarus. |
