768 



CRUSTACEA. 



notions of the existence and of the resistance 

 of the bodies with which it finds 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 antennae, 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 antennae 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 antennae, 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 oesophagus ; but the dispo- 

 sition of the parts there presents no 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 



Fig. 396. __ 



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 antennae are 

 certainly not it, as M. de Blainville imagined ;* 

 and the opinion of M. Rosenthal,f who ascribes 

 the function to a cavity which he discovered 

 at the base of the first pair of antennae, 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 (Jig. 396) 

 is found in the inferior surface 

 of the head, behind the an- 

 tennae of the second pair, or 

 upon the first basilar articu- 

 lation of these antennae them- 

 selvesj (Jig. 396, a). It con- 

 sists in the River-crab of a small 

 bony tubercle pierced at its 

 summit by a circular opening, upon 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. 



The membrane in the greater number of the 

 Brachyura is replaced by a small moveable os- 

 seous disc, which in the Maja and some others 

 presents a pretty broad bony plate (fig. 397) at 



Fig. 397. 



A uditwy disc of the Maja Squinado separated from 

 the rest of the apparatus. 



its posterior edge, detaching itself at right 

 angles and running upwards towards the glan- 

 dular organ already mentioned. Near its 

 base this lamellar prolongation is pierced 

 with a large oval opening, over which there 

 is stretched a thin and elastic membrane 

 which might be named the internal auditory 



* Principes d'anatomic comparee, t. i. p. 338 et 

 339. 



t Roll's Archiv. und Treviranus's vermischtc 

 Schriften, 2ter Band. 2tes Heft. 



J Minasi, Dissertazioni di timpanetti del'udito 

 scoperti nel Granchio paguro. Scarpa, De structura 

 fenestrae rotunda;, &c. Anat. observ. 4to. Mutin. 

 1772; Anat. Disq. dc Aiiditu ct Olfactu, fol. Ticin. 

 1789. Cuvier, Lemons d'anatomie compares, t. ii. 

 Milne Edwards, Histoire natuielle dcs Crustaccs, 

 t. i. p. 123. 



