The first ring presents no appendages except in the very highest Crustaceans, and even then they are simple in their composition, and never exhibit more than the stem, which arises from a more remote check to their development dating from about the conimencement of their embryonic evolution ; these are the ocular pe duncles.
The second and third pairs of extremities constitute tbe antenna. These are wanting in a certain number of the inferior species, and even in those among which they occur, they vary considerably in their structure : they may for instance present one only, or two, or the whole of the three elements of which we have spoken. But as the three first pairs of ap added to this first pair, and these are desig nated jaws or maxilla. In the majority of instances, moreover, the three succeeding pairs assist the three preceding; and as they are frequently more especially apportioned to loco motion, the two last in particular, whilst in some cases they serve for the two functions at one and the same time, they have been de signated by anatomists and naturalists the maxillary limbs (pieds-machoirs): these we shall describe when we come to speak of the apparatus of digestion.
As to the five pairs which we have already mentioned as essentially ambulatory (see fig. 382), they present in the Brachyura no more than a simple stem, composed of six articulations ; whilst in the Astacus and allied genera, we find a flabell.iform appendage or whip, dedicated especially to the purposes of respiration, and in the Pene the three sorts of appendages existing simultaneously. By and-by, when speaking of respiration, we shall see how it happens that in a great number of these animals the whip ofthe thoracic extremities assumes a vesicular structure, and becomes the organ of this important function.
The same peculiarity is observed in the appendages of the abdominal extremities of a great number of species; but among the members of the most elevated tribes, these appendages are but very slightly developed, and appear to have no other use than to attach the eggs along the in ferior surface of the abdomen.
pendages belong especially to the function of sensation, and as we shall have to revert to these at a later period, and give an ample de scription of their structure, we shall not enter upon this subject farther at present.
The eleven pairs of appendages which suc ceed are variously apportioned between the functions of digestion and locomotion, to which last the five hindmost pairs are entirely dedi cated in the Decapods. In other Crustacea, again, the first pair only is set apart in an especial manner for the office of mastication, all the others then serving for locomotion, and this pair is in consequence very generally de scribed under the name of mandibles ; very commonly one and even two other pairs are We shall not at present enter upon the con sideration of the forms of the thoracic and abdominal extremities, having it in view to take up the subject when we come to examine these appendages as the organs of prehension, and as fulfilling important offices in locomotion.
Before quitting the study of the tegumen tary skeleton, to go on to that of the extre mities considered especially as the organs of locomotion, we think it necessary to say a few words upon the moldt or process by which the tegumentary covering of the whole of the Crus tacean is cast off and renewed.
The necessity for this operation is a con sequence of the very nature of the envelope : like every other epidermic covering, the pro duct of secretion, the shell of the Crustacea is closed in on every side, and can only increase in thickness, so that all growth would be pre vented in the body of these animals were they denied the power of freeing themselves from time to time of their prison. Accordingly they
have this power; and as might have been ex pected the shell is cast by so much the more frequently as the animal is younger, inasmuch as the growth is then most rapid ; as many as eight changes of the tegurnentary envelope have been observed to take place in the course of seventeen days in the young Daphnia; whilst in adult Crustacea the change is not in general effected oftener than once a year.
lleaumur watched the phenomenon through its whole course, and has noted it with all its details as it occurs in the Astacus fluviatilis.l. It takes place in this species towards the end of summer or beginning of autumn. A few days of fasting and sickness precede it, during which the carapace becomes loosened from the corium to which it adhered, and which im mediately begins to secrete a new one, soft and membranous at first, but soon becoming harder and harder, and finally completely cal careous. In this way the animal before long finds itself free from all connexion with its old envelope, and it has only to make its escape. This last operation is announced by symptoms of inquietude. The creature rubs its legs one against another, and then throwing itself upon its back begins to shake itself, puffs itself out, so as to tear the membrane which connects the carapace with the abdo men, and to raise the carapace itself. After sundry intervals of rest and agitation of longer or shorter dumtion, the carapace is raised com pletely ; the animal extricates its head, its eyes, and its antennw. The operation of freeing its extremities appears to be the most difficult, and would even be impossible did not the solid covering of these parts split longitudi nally; but in spite of every assistance, it not unfrequently happens that the animal leaves one or two of its limbs impacted within the old sheath, and occasionally even perishes through inability to escape completely from its shell. The abdomen is the last division of the body which clears itself of the old enve lope. All the parts of the tegumentary ske leton which had only been separated from one another, without however having undergone any softening, or fracture, or separation, fall one upon another in resuming their old posi tions, so as to represent the complete external form of the creature with the whole of its solid internal as well as external parts ; even the eyes, the antennee, and the thoracic cells formed by the sternal and epimeral apodemata, may be distinguished. The operation now described does not in general occupy more than half an hour in the performance; and only two or three days, or even no more than four-and-twenty hours are required to convert the soft and membranous envelope with which the corium or naked body of the animal is surrounded, into a firm calcareous covering similar to the one which has just been got rid of. The new envelope presents the same appendages as the former one, even the same hairs ; but these, instead of being con tained within the old ones, as Reaumur ima gined, exist ready formed in the new envelope, but turned in towards the interior, like the fingers of a glove turned in upon themselves.