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.
(994). Having thus found that the annuli, or rings, which compose the annulose skeleton may be detected even in the most compactly formed Crustacea, it remains for us to inquire, in the next place, what are the principal modifications observable in the articulated appendages developed from the individual segments. This inquiry is one of considerable interest, inasmuch as it goes to prove that, however dissimilar in outward form, or even in function, the limbs of Crustaceans are mere developments of the same elements, which, as they remain in a rudimentary condition or assume larger dimensions, become converted into instruments of sensation, legs, jaws, or fins, as the circumstances of the case may render needful. In the lower, or more completely annulose forms (figs. 195 and 198), these members are pretty equally developed from all the segments of the body, and are subservient to locomotion, being generally terminated by prehensile hooks, or provided with fin-like expansions; but as we advance to the more perfect genera, the limbs assume such various appearances, and become convertible to so many distinct uses, that they are scarcely to be recognized as consisting of similar elements modified only in their forms and relative proportions.
Fig. 197. Limulus Polyphemus.
To notice all the varieties which occur in the extensive class before us, would be to weary the reader with tedious and unnecessary details; we shall therefore select the Decapod* division of these animals, as abundantly sufficient for the illustration of this part of our subject. This division, which includes the most highly organized forms, has been divided by writers into three extensive families: the Macroura, or Swimming Decapods; the Anomoura, which inhabit the empty shells of Mollusca; and the Brachyura, or short-tailed species, of which the Crab is a familiar specimen. If we take the common Lobster as an example of the first of these groups, we shall find that there are five pairs of articulated limbs placed upon each side of the mouth, which are evidently adapted to assist in seizing and conveying into the stomach substances used as food. These singular organs, although entitled to be considered as jaws so far as their use would indicate the name belonging to them, are no less obviously merely modifications of articulated feet; and the term foot-jaws has now, by common consent, become the appellation by which they are distinguished.
(995). The pair of legs which succeeds to the remarkable members last referred to is appropriated to widely different offices. The organs in question are developed to a size far surpassing that attained by any of the other limbs, and are endowed with proportionate strength. Each of these robust extremities is terminated by a pair of strong pincers (chelae); but the two are found to differ in their structure, and are appropriated to distinct uses. That of one side of the body has the opposed edges of its terminal forceps provided with large blunt tubercles, while the opposite claw is armed with small sharp teeth. One, in fact, is used as an anchor, by which the Lobster holds fast by some submarine fixed object, and thus prevents itself from being tossed about in an agitated sea; the other is apparently a cutting instrument for tearing or dividing prey.
(996). To the chelce succeed four pairs of slender legs, scarcely at all serviceable for the purposes of locomotion; but the two anterior being terminated by feeble forceps, they become auxiliary instruments of prehension.
(997). The articulated appendages belonging to all the abdominal segments are so rudimentary that they are no longer recognizable as assistants in progression; and it is at once evident, when we examine the manner in which the Macroura use their tails in swimming, that the development of large organs in this position would materially impede the progress of animals presenting such a construction; the false feet, as these organs are called, are therefore merely available as a means of fixing the ova which the female Lobster carries about with her attached beneath her abdomen.
* So called from the circumstance of their having five pairs of limbs so largely developed as to become ambulatory or prehensile organs.
(998). The tail is the great agent of locomotion in all the Macroura or large-tailed Decapods; and for this purpose it is terminated by a fin, formed of broad calcareous lamellae, so arranged that, while they will close together during the extension of the tail, and thus present the least possible surface to the water, they are brought out to their full expansion by the down-stroke of the abdomen; and such is the impulse thus given, that, as we are credibly informed, a Lobster will dart itself backwards to a distance of eighteen or twenty feet by one sweep of this remarkable locomotive instrument.
(999). If we now pass on to the consideration of the Anomourous Decapods, we find that the external organs above enumerated, although existing in precisely similar situations, are so far modified in their construction and relative proportions as to become suited to a mode of life widely different from that led by the members of the last division. The Anomoura, as their name imports, have tails of very unusual conformation. Instead of being encased in a hard coat of mail, as in the Macroura, the hinder part of the body is soft and coriaceous, possessing only a few detached calcareous pieces - analogous, it is true, to those found in the Lobster, but strangely altered in structure.
Fig. 199. Hermit Crab.
(1000). These animals (fig. 199), usually known by the name of Soldier Crabs, or Hermit Crabs, frequent level and sandy shores, and, from their defenceless condition, are obliged to resort to artificial protection. This they do by selecting an empty turbinated shell of proportionate size, deserted by some gasteropod mollusk, into which they insinuate their tail, and, retreating within the recesses of their selected abode, obtain a secure retreat, which they drag after them wherever they go, until, by growing larger, they are compelled to leave it in search of a more capacious lodging. The wonderful adaptation of all the limbs to a residence in such a dwelling cannot fail to strike the most incurious observer. The chelae, or large claws, differ remarkably in size; so that, when the animal retires into its concealment, the smaller one may be entirely withdrawn, while the larger closes and guards the orifice. The two succeeding pairs of legs, unlike those of the Lobster, are of great size and strength, and, instead of being terminated by pincers, end in strong pointed levers, whereby the animal can not only crawl, but drag after it its heavy habitation.