This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Behind these locomotive legs are two feeble pairs, barely strong enough to enable the Soldier-Crab to shift its position in the shell it has chosen; and the false feet attached to the abdomen are even still more rudimentary in their development. But the most singularly altered portion of the skeleton is the fin of the tail, which here becomes transformed into a kind of holding apparatus, by which the creature retains a firm grasp upon the bottom of its residence.
Fig. 200. Swimming Crab.
(1001). In the Brachyura, or Crabs, we have at once, in the concentration observable in all parts of the skeleton, an indication of its being formed for progression on land, or at least for creeping at the bottom of the sea. The tail, the great instrument of locomotion in the Lobster, is here reduced to a rudiment, and the fin at its extremity entirely obliterated; the chelce still continue to be the most powerfully developed of the extremities; while the legs, the principal locomotive agents, are either terminated by simple points, as in those species which are most decidedly terrestrial in their habits, or else, in the Swimming Crabs, the posterior pair become expanded into flattened oars useful in natation (fig. 200).
(1002). From the extreme hardness and unyielding character of the tegumentary skeleton in Crustaceans, a person unacquainted with the history of these animals would be at a loss to conceive the manner in which their growth could be effected. In insects we have seen that all increase of size occurs prior to the attainment of the perfect condition, and expansion is provided for by the moults or changes of skin which take place during the development of the larva; but the Crustacean, having acquired its mature form, still continues to grow, and that until it acquires in many instances a size far larger than that which any insect is permitted to arrive at.
(1003). The plan adopted in the case before us, whereby growth is permitted, is attended with many extraordinary phenomena. At certain intervals the entire shell is cast off, leaving the body for the time unfettered indeed as regards the capability of expansion, but comparatively helpless and impotent until such time as a new shell becomes secreted by the dermis, and by hardening assumes the form and efficiency of its predecessor.
(1004). We are indebted to Reaumur*, who watched the process in the Cray-fish (Astacus fluviatilis), for the first account of the mode in which this change of shell is effected. In the animal above mentioned, towards the commencement of autumn, the approaching moult is indicated by the retirement of the Cray-fish into some secluded position, where it remains for some time without eating. While in this condition, the old shell becomes gradually detached from the surface of the body, and a new and soft cuticle is formed underneath it, accurately representing, of course, all the parts of the old covering which is to be removed; but as yet little calcareous matter is deposited in the newly-formed integument. The creature now becomes violently agitated, and, by various contortions of its body, seems to be employed in loosening thoroughly every part of its worn-out covering from all connexion with the recently-secreted investment. This being accomplished, it remains to extricate itself from its imprisonment - an operation of some difficulty; and when the nature of the armour to be removed is considered, we may well conceive that not a little exertion will be required before its completion.
As soon as the old case of the cephalothorax has become quite detached from the cutis by the interposition of the newly-formed epidermic layer, it is thrown off in one piece, after great and violent exertion; the legs are then withdrawn from their cases, also after much struggling; and, to complete the process, the tail is ultimately, by long-continued efforts, extricated from its calcareous covering, and the entire coat of mail which previously defended the body is discarded and left upon the sand. The phenomena which attend this renovation of the external skeleton are so unimaginable, that it is really extraordinary how little has been done towards elucidating the nature of the operation. The first question which presents itself is, how are the limbs liberated from their confinement? for, wonderful as it may appear, the joints even of the massive chelae, of the Lobster do not separate from each other, but, notwithstanding the great size of some of the segments of the claw, and the slender dimensions of the joints that connect the different pieces, the cast-off skeleton of the limb presents exactly the same appearance as if it still encased the living member.
The only way of explaining the circumstance is to suppose that the individual pieces of the skeleton, as well as the soft articulations connecting them, split in a longitudinal direction, and that, after the abstraction of the limb, the fissured parts close again with so much accuracy that even the traces of the division are imperceptible. But this is not the only part of the process which is calculated to excite our astonishment: the internal calcareous septa from which the muscles derive their origins, and the tendons whereby they are inserted into the moveable portions of the outer shell, are likewise stated to be found attached to the exuviae; even the singular dental apparatus situated in the stomach, of which we shall speak hereafter, is cast off and re-formed! And yet, how is all this accomplished? how do such parts become detached? how are they renewed? We apprehend that more puzzling questions than these can scarcely be propounded to the physiologist; nor could more interesting subjects of inquiry be pointed out to those whose opportunities enable them to prosecute researches connected with their elucidation*.
* Mem. de l'Acad. des Sciences, 1718.