This animal, in the above respect, being so completely insulated, it seems, as if in its means of entrapping its prey it was formed upon a plan not connected with that of any other Molluscan, but quite sui generis: probably, were we acquainted with the animals belonging to what are deemed fossil Cephalopods, we should find the hiatus vastly narrowed. In this instance we see clearly that adaptation of means to an end which distinguishes all the works of the Creator; the striking variation which this creature exhibits from the oral apparatus of its Class, is evidently connected with the kind and circumstances of the animals which it is commissioned to keep within their proper limits; its mandibles, or beak, indeed, resemble those of the other Cephalopods, indicating that its prey are covered with solid integuments, requiring great force to crush them; but the other oral organs, and its snail-like foot, as we see, indicate that they are not of a kind that can easily escape from their assailants. Two objects seem to have been principally in the mind of the Almighty planner of the universe of beings: one seems to have been the concatenation of all subsistences, seriatim and collaterally, into one great system; and the other, so to order and vary the structure of each individual that it may be duly fitted to answer a certain end, and produce a certain effect upon such and such points of that system, and this in such a way that these effects, though diverse, might not be averse, but proceed, if I may so speak, in the same direction. Thus, in the subject before us, the general commission given to the Cephalopods, is to assist in reducing the armed population of the ocean within certain limits, and to all are given instruments and organs, varying indeed in their structure, but proper to enable them to effect this purpose; all, however, concurring to bring about a common and connected object, and one taking one department and another another. The tentacles of the Univalve Molluscans, for the headless animal of the Bivalves has no such organ, are neither used for locomotion nor prehension, and therefore seem to have no claim to a place in the present chapter. But as they are clearly the analogues of the tentacles of the animals we have been considdering, and though not prehensory, are certainly exploring and sensiferous organs, which are probably connected with prehension, I shall make a few observations upon them. They vary in their number, some having none,1 others only two; others 1 Chiton. 2 Cypræa. Voluta. PLATE VI. FIG. 1. b. again four1 and lastly, others six. They are without articulations, though they sometimes exhibit an annulated appearance: they are also often retractile, and in the snail and slug they form a hollow tube, which can be inverted like the finger of a glove; in others they appear to be composed of longitudinal fibres, intersected by annular ones, which render them capable of great extension. In form they are either filiform, setaceous, or conical; but in the remarkable genus Laplysia, or the Sea-hair, the upper pair are shaped like the ears of the animal from which they take their name. Their sense of touch is much more delicate than that of the rest of the body. They are intimately connected with what are usually deemed the organs of sight of the Univalve Molluscans, which in some genera they seem to inclose. Some of these eyes are placed, in the form of a black pupil, at the summit of the tentacle, which surrounds them as the iris does the pupil of the perfect eye; in others they are imbedded in the middle of that organ, and in others at its base; in some, as in the Sea-ear, they are seated in a separate footstalk. In many of the carnivorous species the pupil is surrounded by an iris," which seems to indicate that the tentacles perform, in some sort, the functions of that part of the eye. The upper pair of tentacles in the Molluscans seem analogues of the antennae of Condylopes, and the lower pair of their feclers; and the functions for which the Creator has formed and fitted both are probably not very dissimilar. The extreme irritability of the tentacles of snails and slugs is evident to every one who observes their motion: at the approach of a finger they are immediately retracted; they therefore give notice to the animal of the approach of danger, so as to provide against it, and when necessary to withdraw itself into its shell: the eyes, from their situation in many of them, supposing them to have a greater range and power of vision than they appear to have, cannot direct them in the choice of their food, in these their lower tentacles may have this office. Snails and slugs, we also know, issue forth from their places of concealment when the earth is rendered moist enough, by showers, for them to travel easily over its surface; so that they must be endued with some degree of aëroscepsy, of which probably these delicate organs are the instruments. 1 Helix. Limar. 2 Clio. The tentacles in this genus are retractile, and when retracted form two tubercles, which make the head appear bilobed. 3 Voluta Ethiopica, PLATE VI. 5 PLATE VI. FIG. 1, a. 4 Haliotis. Whether the barbs appended to the mouths of many fishes, as the barbel, the Siluridans,' and the Fishing-frog, may be regarded as a kind of tentacle cannot be certainly affirmed, but from their proximity to the mouth, it seems most probable that they exercise some function connected with the procuring of its food. Cuvier regards them as a kind of tactors, and they also present some analogy to antennæ and palpi. In many of the Annelidans, tentacles of the present description are found not only in the vicinity of the mouth, but also upon the pedigerous segments of the body, and appear to be equally used in exploring objects. I shall next consider some tentacular organs, which differ from those we have been considering in being more or less jointed. These, on that account, have been considered as a different class of organs, and by many have been denominated cirri or tendrils, or more properly, by Savigny, tentacular cirri. I have before described organs of this kind in my account of the Cirripedes, by which it appears that they are employed for the same purposes as the tentacles of the polypes. Under this head also the antennæ of Crustaceans and insects may be noticed, which seem, as I have lately observed, analogous to the tentacles of the Molluscans, and the barbs of fishes; in some instances, indeed, they are used instead of the fore-legs. The reason why their structure differs from the soft, inarticulate tentacles above described, at least in most cases, appears to be the different nature of the integuments of the animal, which being incased in a kind of coat of mail, it seems requisite that both its locomotive and oral organs should be similarly defended, and in this case, unless they had been jointed, they would have lost their flexibility, and so could not have exercised the functions assigned to them by their Creator. It may, perhaps, be objected that the shell of the snail is nearly as hard as the crust of the lobster; but when we consider that the former, when moving, can thrust forth the greatest part of its soft body, as it were from a house, while the crust of the other is really its skin, this objection seems to vanish. Suckers.-The organs I am next to consider, acetabula, or suckers, are, in many cases, so intimately connected with tentacles, as to form the most essential feature of them, without which they can be of no use. In fact, in the Cephalopods, 1 PLATE XII. FIG. 1. 2 Lophius. PLATE XIII. FIG. 2. 4 See above, p. 189. they bear the same relation to the organ just named that the hand or foot do to the arm or leg, or the fingers and toes to the hand, in higher animals: they are the part by which the animal takes hold of what it wants to seize; and by the alternate fixing and unfixing of which, upon a solid substance, it moves from place to place. A sucker1 may be defined-An organ, by which an animal is enabled to create a vacuum between it, (the organ,) and any surface on which it rests, so as to produce a pressure of the atmosphere upon its upper part, and thus causing it to adhere firmly. Cuvier, speaking of the suckers of the Cephalopods, thus describes their action. When the animal approaches one or more of its suckers to a surface, in order to apply it more intimately, it presents it flattened; when it is fixed to it by the perfect union of the surfaces, it contracts its sphincter, which produces a cavity, in the centre of which a vacuum is formed. By this mechanism, the sucker attaches itself to the surface with a force proportioned to its diameter, and to the weight of the column of water or of air of which it is the base. This force, multiplied by the number of suckers, gives that with which the whole or part of the legs attaches itself to the body, so that it is more easy to tear the legs, than to separate them from the object which the animal wishes to retain. In some cases, the action of the suckers, as suckers, seems not sufficient for the animal's purposes, and claws are superadded. This structure is to be found in the suckers of the animal that fixes itself to the gills of the bream, the Diplozoon before described, and to those of some Cephalopods a stout claw is added. When we consider the nature and predatory habits of those Cephalopods whose tentacles are furnished with suckers, often pedunculated, on that side which is prone when the animal moves, we shall at once see the reason that this change, from the more common Molluscan structure of an expansile foot, took place, for had their principal locomotive and prehensory organ been of this description, or different from what it is, their motions must necessarily have been so slow, and their powers of prehensson so weak, that they could never have overtaken and captured, and maintained their hold of the well-defended and formidably armed Crustaceans, which are their destined prey. Uncouth, therefore, and mis-shapen and monstrous, as 1 Suckers are denominated scientifically Acetabula, and Cotyle, or Cotyloid 3 See Appendix. processes. 2 Anat. Comp. i. 410. Roget, B. T. i. 260. these animals, at the first glance, appear, we see that in these organs, and, doubtless, in all others, they are exactly fitted to answer the end, and fulfil the purposes of Divine Providence in their creation. The suckers of the Diplozoon exhibit a complex structure in aid of its powers of suction, not easily developed and understood. Dr. Nordmann supposes, that though the animal could attach itself strongly by these organs, additional means were necessary to render its attachment sufficiently firm; and that. therefore, while it is fixing itself by the suckers, it requires the aid of the apparatus of hooks, or claws and arches, to keep itself from being misplaced.1 The Class of Annelidans exhibits a great variety of locomotive organs, amongst the rest, in the last Order, we find suckers, these being the principal organs for motion of the Hiru dineans or leeches, the animals of which Order, however, M. Savigny is disposed to think are essentially distinct from the rest of the Annelidans, on account of their want of sete or la teral bristles. The oral sucker of that division of the animals I am considering, to which the common leech belongs, is distinguished from the anal one by being formed of many segments. whereas the latter consists of only one. Their motions, by means of these suckers,and the annular structure of their bodies, I have before sufficiently described. Their suckers also enable them to lay hold of any aquatic animals that come in their way, especially the oral one, which, once fixed, they soon make an entry and begin to imbibe its blood. We see, in this, the reason why their Maker, instead of bristles for locomotion, has given them organs by which they can not only move from one place to another, but also fix themselves firmly to their prey. I shall next advert to a kind of sucker which really becomes both the hand and foot of the animals that bear them. I allude to those of the Echinoderms, described on a former occasion, in which the ampullaceous part within the shell presents the first outline of a shoulder or thigh, the exerted extensile part that of an arm or leg, and the dilated part with which the animal seizes its prey or walks, the hand or foot; the two first constituting the tentacle, and the last the sucker. I have, on a former occasion, given some account, under 1 See Nordmann, i. 61. t. v. f. 3, 4, 5. 2 Sanguisuga medicinalis. Sav. 4 See above, pp. 108, 111. PLATE III. FIG. 5. 3 See above, p. 181. |