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.
"The new limb is formed within the shell, where it lies folded up until the next moult, when it appears as a part of the new skeleton, the sac-like membrane which protected it being cast off with the old shell; and the restored member is larger or smaller, in accordance with the length of time which may exist between the amputation of the limb and the shedding of the skin. The condition in which the limb is at that time remains permanent until the next moult, when the whole insects, are equally applicable to the animals composing the class before us; for in the Crustacea, although we are compelled to admit the possession of the above facidties, we are utterly ignorant of the mode in which they are exercised; therefore it would be only an unprofitable waste of time to enter at any length into a discussion from which no satisfactory conclusions are, in the present state of our knowledge, to be deduced.
(1040). But the most remarkable part of the phenomenon remains to be noticed: - After this extraordinary amputation has been effected, another leg begins to sprout from the stump, which soon grows to be an efficient substitute for the lost extremity, and gradually, though slowly, acquires the pristine form and dimensions of its predecessor. A beautiful example of this curious mode of reproducing a lost organ is preserved in the Museum of Comparative Anatomy in King's College, London, in which the new limb (one of the cheliferous claws) has already attained the form of the old chela, but still remains soft and uncovered by calcareous integument. The process of reproduction is as follows: - The broken extremity of the second joint skins over, and presents a smooth vascular membrane, at first flat, but soon becoming conical as the limb begins to grow. As the growth advances, the shape of the new member becomes apparent, and constrictions appear, indicating the position of the articulation; but the whole remains unprotected by any hard covering until the next change of shell, after which it appears in a proper case, - being, however, still considerably smaller than the corresponding claw on the opposite side of the body, although equally perfect in all its parts.
(1041). Mr. H. D. S. Goodsir has shown* that in the Lobster this regenerative faculty does not reside at any part of the claw indifferently, but in a special locality, situated at the basal end of the first joint of each of the legs. This joint is almost filled by a mass of nucleated cells surrounded by a fibrous and vascular band; and other nucleated cells intervene between this vascular band and the outer crust. The vessels of the band pass onwards for about half an inch, and return upon themselves, forming loops. When a claw is broken, or otherwise injured or disabled, the Lobster, or Crab, by a violent muscular effort casts it off at the transverse ciliated chink, or groove, which indents the reproductive segment. The new claw is developed by the multiplication of cells, which soon become divided into five groups, answering to the five joints of the future limb; these nascent joints are folded upon each other in the Crab, but extended in the Lobster; in both, they are at first enveloped in a sac formed by the distended cicatrix; the budding limb ultimately bursts this cicatrix, and its growth is rapidly completed.
A great proportion of the reproductive cells contained in the basal extremity of the injured limb is made use of in the production of the new limb; but a mass of them is retained unchanged at the basal joint, and is ready to renew the reproductive process when needed. In the lower Crustaceans such groups of cells are found at more numerous joints.
(1042). The observations made in a former chapter relative to the organs by which the senses of touch, taste, and smell are exercised in creature again advances in size, but the new limb more rapidly than the remainder of the animal, until it attains its full relative proportions." - Ann. and Mag. Nat. Hist. 1851, vol. vii. p. 300. * Vide Owen on Parthenogenesis, p. 48.
(1043). The eyes of Crustaceans are of three kinds - simple, agglomerated, and compound.
. (1044.) The simple eyes (ocelli, stemmata) resemble those of Spiders, and, like them, are said to consist of a cornea, a spherical lens, a gelatinous vitreous humour, a retina, and deeply-coloured choroid, all occupying their usual relative positions. These eyes never exceed two or three in number.
(1045). In the agglomerated eyes, such as those of Daphnia (fig. 212), the organ seems to be composed of a number of simple eyes placed behind one common cornea; such eyes are moveable; and in the animal depicted in the figure, the muscles acting upon the visual apparatus, which in this case is single, are arranged so as to form a cone, the base of which is formed by the eye, and may be distinctly seen under a good microscope.
(1046). The compound eyes appear to be constructed upon the same principles as those of insects. The corneae are extremely numerous, and in general hexagonal; but sometimes, as in the Lobster, they are square. The vitreous humours equal the corneae in number, and behind each of these a distinct retina would seem to be expanded. The compound eyes of Crustaceans have not, however, as yet been examined with the same patient diligence as those of the Cockchafer; so that, as relates to their minute anatomy, much is still left to conjecture and uncertainty. One peculiarity connected with these organs is, that in the two highest orders of Crustacea, hence called Podophthalmia, the eyes are placed at the extremity of moveable pedicles articulated with the first cephalic ring of the external skeleton, and thus they may be turned in various directions without moving the whole body at the same time. This provision was not required in insects, owing to the mobility of the head in those animals, but is absolutely indispensable in the case before us, where, the head and thorax being consolidated into one mass, the extent of vision commanded by sessile eyes would have been exceedingly limited, and inadequate to the security of creatures exposed to such innumerable enemies.