B. Organs of the senses.—Do the five senses exist, and to what degree of development have they attained in the Crustacea ? Such is the question we have now to consider, and which we shall sometimes find ourselves in a condition to answer from the simple inspection of the various organs of special application.
Thus we discover almost at once that the sense of general touch is obtuse, and can convey to the animal no other but confused notions of the existence and of the resistance of the bodies with which it fin& itself in immediate relationship by its external surface. To be satisfied of this, it is enough to consider for a moment the hard and unyielding nature of the general tegumentary envelope over every point of the body except the articulations,— parts which on other grounds are obviously inadequate to exercise any sense whatever.
Nevertheless, in front of the head there are certain special organs which all the observa tions I have had an opportunity of making upon the organization of these animals lead me to regard as parts more particularly destined to be the seat of the sense of touch. These organs are the antennze,—those slender fila ments, possessed of a great degree of flexibility, of motility, and of sensibility. _M. de Blain ville was led to regard these organs as the seat of the sense of smell ; but direct and conclusive experiment has satisfied us that the destruction of the antennw has no influence whatever on the exercise of the sense of smell : and we are on the same grounds' in duced to believe them destined to the exer cise of the sense of touch of considerable delicacy, unless we would imagine them as the instruments of some quite peculiar sense the existence of which would be purely hypothetical.
The number and disposition of these organs varies extremely. Some of the Crustaceans at the very bottom of the series are wholly without antennw, or are furnished with them in a merely rudimentary state. Some species have no more than a single pair; the normal number, however, is two pairs. In speaking of the tegumentary skeleton, we have said to which of the rings these appendages belong; we shall only say farther here, that they may be inserted on the superior or on the inferior surface of the head according to the respective development of the different pieces of which this segment is composed. They do not differ less widely in their form and composition, and under this double point of view present modi fications analogous to those which we have specified as occurring in the extremities.
The Crustaceans, like almost all other animals, make a selection of matters in especial relation ship with the state of their organs of nutrition ; they must therefore be endowed with the sense of taste. With reference to the seat of this faculty, which perchance is the mo dification of sensibility the least remote from the sense of touch, it appears to reside in the Crustacea, as it does obviously in the majority of animals, in that portion of the tegumen tary membrane which lines the interior of the mouth and cesophagus ; but the dispo sition of the parts there presents DO peculiarity worthy of especial notice.
The Crustaceans perceive the existence of bodies at a distance by the medium of odorous particles emitted from these bodies. Many of the known habits of these animals, and the certainty with which they are attracted by baits placed in close traps from which the light is excluded, do not allow us to entertain any doubts upon this point; but we are reduced to conjecture when we are required to point out the precise seat of the organ of smell. The horny appendages named antennm are certainly not it, as M. de Blainville imagined ;1' and the opinion of M. Rosenthal,1- who ascribes the function to a cavity which he discovered at the base of the first pair of antennw, requires to be supported by direct experiment.
Hearing, at least in a great number of species, resides in a particular apparatus per fectly well known. It (fig. 396) is found in the inferior surface of the head, behind the an tennw of the second pair, or upon the first basilar articu lation of these antennze them selves$ (fig. 396, a). It con sists in the River-crab of a small.
bony tubercle pierced at its summit by a circular opening, urfm. which is stretched a thin elastic membrane, which Scarpa has compared to that of the tym panum, or of the fenestra ovalis of the ves tibule in the higher animals. Behind this membrane there is a membranous vesicle filled with fluid, into which a branch of the antennary nerve is observed to plunge. Above this organ there is another of a glandular appearance, the intimate relations of which with the apparatus we have just described might lead to the belief that it was not un connected with the sense of hearing. In the Palinurus it communicates with an opening which is pierced through the centre of the membrane that closes the auditory tubercle in front.