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.
Fig. 208. Female generative organs of Astacus Jluviatilis: a,bb, ovaria; c, oviduct; d, external termination of the right oviduct; e, escaped ovum.
(1064). In a more advanced stage of growth the creature assumes a totally different shape (fig. 210), under which form it has been known to naturalists by the name of Megalopa. The eyes become pedunculated, the cephalothorax rounded, the tail flat and provided with false feet, and the chelae and ambulatory extremities well developed.
Fig. 209. Metamorphosis of Crab.
Fig. 210. Metamorphosis of Crab.
* Sulla Generazione dci Pesci e dei Granchi. 4to. Naples, 1787.
(1065). A subsequent moult gives it the appearance of a perfect Crab; and then only does the abdomen become folded under the thorax, and the normal form of the species recognizable (fig. 211).
Fig. 211. Metamorphosis of Crab.
(1066). Notwithstanding the diversity of form under which the young Crab presents itself at different phases of its growth, examples of which we have here figured, it would seem, from the observations of Mr. C. Spence Bate*, that the progress made towards the mature condition is not by any sudden metamorphosis, but by a series of moult-ings similar to those which take place in the adult, and that with each successive moult there is a corresponding degree of progress in its development. But the amount of change at each moult is so little that it gives to the animal but a very small degree of difference in its general appearance; and it is only by a comparison of the earliest form with the last, and that without any consideration of the intermediate stages in its growth, that the idea of a true metamorphosis in Decapod Crustacea has arisen. There are, in fact, six or seven well-marked stages or forms that the growing animal passes through in its progress to maturity; and each of these is linked to the preceding as well as to that which follows by a succession of changes that are but just appreciable.
In the Branchiopod Crustacea (so called from this circumstance), the legs used in swimming would appear to be converted into broad-fringed lamellae, so thin that they perform the office of branchiae, and render needless the existence of other instruments of respiration. In Daphnia, for example (fig. 212), a creature common in every stagnant pool, the body is contained, as it were, between two corneous plates, open along their inferior edge. Through this transparent envelope the legs may be perceived in constant movement: and from the extreme delicacy of the covering that invests them, they evidently present to the surrounding medium a surface of sufficient extent for the purpose of exposing the blood to its action, thus rendering them efficient substitutes for branchiae, while, at the same time, their movements ensure a perpetual renovation of the water in contact with them; so that, as a necessary consequence, the respiratory process will be accomplished with greater completeness in proportion as the exertions of the animal become more vigorous. In the Crustacea, indeed, we have many interesting and beautiful examples of the connexion between the respiratory and locomotive organs. The amount of respiration must necessarily be equivalent to the expenditure of muscular energy; and a more elegant manner of ensuring an exact correspondence between the one and the other, than that adopted, could scarcely be imagined; for, by appending the branchiae to the locomotive agents themselves, the more actively the latter are employed, the more freely will the former receive the influences of the aerated water in which they are immersed.
* Phil. Trans. 1848.
Fig. 212. Daphnia.
(1068). In the Squilla, which swims by means of the movements of its broad tail, it is the false feet beneath the abdominal segments that become branchial organs; and these, being expanded into broad and vascular lamellae, perform the office of gills. In the Squilla, therefore, and similarly-formed genera, the free movement of the tail ensures the full and complete exposure of the respiratory structures to the surrounding element.
(1069). The more minute Branchiopods, or Entomostraca, as they are called by zoologists, offer, in their mode of reproduction, several remarkable variations from what has been described above; and a brief account of their most interesting peculiarities is therefore still wanting to complete this part of our subject. These little creatures, in fact, seem to form a transition between the class we are now considering and the Epizoa, which many of them resemble so nearly, that they are still confounded together by many authors. The female Entomostraca frequently carry their ova in two transparent sacculi attached to the hinder part of the body; and it is in these egg-bags that the oviducts terminate; so that the ova, as they are formed, are expelled into the singular receptacles thus provided. Without such a provision, indeed, it would be difficult to conceive how the ova could possibly remain attached to the parent, as they far surpass in their aggregate bulk the size of her entire body, and therefore could not by any contrivance be developed internally without bursting the crustaceous covering that invests the mother.
Jurine*, Ramdohr1, and other authors have carefully watched the generative process in several genera, and brought to light many important and curious facts connected therewith. In Cyclops, a species to be met with in every ditch, the impregnation of the ova is undoubtedly effected in the body of the parent; and the eggs, when formed, are expelled into two oval sacs placed on each side of the tail, which Jurine calls external ovaries. The number of eggs contained in these sacs gradually increases; and they exhibit a brown or deep-red colour until a short period before the growth of the embryo is completed, when they become more transparent. In about ten days the eggs are hatched and the young escape; and such is the prodigious fertility of these little beings that a single female will, in the course of three months, produce ten successive families, each consisting of from thirty to forty young ones.