During the progress of the metamorphosis, the nervous system within is undergoing a pro cess of concentration precisely comparable to that which has been noticed in advancing from the lower to the higher classes of ARTICULATA. The ganglia coalesce and become less numerous, the encephalic pair attain a higher development, and as this is accomplished the legs and wings of the mature being sprout from the sides of the segments appropriated to sustain them, enclosed in and defended by cases of cuticle temporarily provided, which constitute the covering of the pupa or chrysalis, until at length, the aggregation of the previously sepa rated ganglia being completed and the brain perfected to the extent required, the pupa-case is thrown off, the newly-formed limbs expand, and the insect, with its newly-acquired limbs, pos sesses an additional system of muscles, which have been developed with their growth, and only arrive at their full state of perfection when the body has ceased to grow, and the genera tive system, having attained its complete pro portions, proclaims the animal mature and able to propagate its species.
The addition of wings, indeed, to the body of flying insects would seem to be a provision specially connected with the diStribution of the progeny to which they are to give birth, and all the phenomena connected with their develop ment and that of the muscular appar itus pro vided for their movements to have relation to this great and closing act of the insect's exist ence. The period of time during which these animals live in their imperfect or wingless state, during which many of them have important offices assigned to them, constitutes, in most cases, by far the longest portion of their lives, and some aquatic larva:, indeed, reside for months or even years in the water under their immature or wingless form, which perish in a few hours after they have been gifted with the means of aerial locomotion. Had they never been furnished with wings, it is abundantly evident that the species of such insects could never have been dispersed beyond the precincts of the pond or the ditch in which the parent had passed her existence, but the brief space allowed them to enjoy life in the winged con dition is sufficient for the achievement of the great object in view, and the Ephemeron and the little Gnat, while they appear to be only sporting out their evening's life amid the sun beams, are, in fact, disseminating their offspring through different localities.
The next class of HOMOGANGLIATE animals comprises the ARACIINIDANS, the Scorpions and the Spiders, animals visibly intended to be destroyers, appointed to keep within due limits the different races of the Insect world, and by assisting in the great work of destruction that is on all sides in progress against them, to prevent their fertility from becoming prejudicial to the other members of the animal creation. The tyrant must neces
sarily be stronger and more sagacious than the victims intended to be subdued, and accord ingly, in the Arachnids, the great law that has hitherto been our guide in tracing the develop ment of the muscular system is carried out one step further. The coalescence of the nervous ganglia and consequent concentration of the skeleton is found in these creatures to be more conspicuous than even among the Insects the n selves: even the head and thorax, which in the last class were distinct from each other, now become fused into one piece, forming the rephalo-lharax of the creatures under considera tion. The limbs and the jaws are thus rendered stronger and more formidable, and the muscles whereby they are wielded atta n the fullest development permitted amongst articulated animals. Among the C RUSTACEA forming the last class of this important sub-kingdom of creation, we find a series of aquatic Articu!ata running parallel as relates to the condition of their muscular system with the terrestrial Arti culata, and exhibiting precisely the same rela tions between the state of concentration of the nervous system, and the degree of efficiency conferred upon their locomotive apparatus. The humblest forms of Crustaceans have all the segments of the body distinct and move able, and, moreover, in their elongated shape resemble the larvx of aquatic Insects. In these the articulated limbs appended to the different segments of the body are extremely feeble, and only adapted to natation ; but pro ceeding upwards in the scale the locomotive members assume a more effective appearance, and the segments supporting them run together, and become consolidated. NVIiilst the muscles of the trunk preponderate in their development, as in the Shrimps and .Macrourous Decapods, the limbs are of secondary importance as instruments of locomotion, and the largely developed tail forms a strong and powerful oar, a means of propulsion best fitted to their nata torial habits; but as we approach the shore and meet with Crustaceans, adapted to a littoral existence, the muscles of the trunk become diminished in importance in proportion as the legs acquire additional strength. The concen tration of the segments of the trunk is carried out to the greatest possible extent in the Bra chyurous Decapods, and the Crabs are thus enabled to leave the sea and prowl about upon the beach, or even to exchange an aquatic for a terrestrial existence; and, as in the case of the Land Crabs, to reside during a greater part of the year at a distance from their native element.