540 



MUSCULAR SYSTEM. 



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 Arachnida, 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 

 cephalo-t/iorajc 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 attain the fullest 

 development permitted amongst articulated 

 animals. Among the CRUSTACEA forming the 

 last class of this important sub-kingdom of 

 creation, we find a series of aquatic Articulata 

 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 larvae 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. Whilst 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. 



It is, however, interesting to observe that in 

 the most highly organized of the CRUSTACEANS, 

 the Bracliyura, the complete centralization of 

 the body and nervous ganglia is effected, as in 

 the case of Insects, in a slow and gradual 

 manner, and that their muscular system under- 

 goes a metamorphosis scarcely less remarkable 

 than that observed among the Insects them- 

 selves. The Grub on first leaving the egg is 

 almost iu the condition of a long-tailed Shrimp, 



and the locomotive limbs scarcely to be recog- 

 nised as being worthy of such a title, being not 

 only rudimentary in their size but exclusively 

 adapted for swimming; and it is only after 

 several times casting its shell and passing 

 through distinct gradations of form, that the 

 muscles of the legs attain the preponderancy 

 over those of the trunk and become strong 

 enough for progression on land. 



The fourth grand division of the animal 

 kingdom, comprising the MOLLUSCA of 

 Cuvier, is characterized by the dispersed con- 

 dition of the nervous ganglia, which, through- 

 out the extensive series of creatures constructed 

 according to this type, are distributed without 

 any symmetrical arrangement in different parts 

 of the body, whence the Mollusca have been 

 named by Professor Grant CYCLO-GANGLIATA, 

 and more recently by Professor Owen HFTERO- 

 GANGLTATA, the latter term being, as we con- 

 ceive, the preferable of the two. In the Mol- 

 lusca the general outline of the body partici- 

 pates, more or less, in the want of symmetry 

 that is so conspicuous in the disposition of the 

 ganglia composing the nervous system, and the 

 muscular apparatus does not exhibit that pre- 

 cision and regularity which is visible among all 

 the Articulata. There is no longer, in fact, 

 any frame-work, but when a shell is present, 

 as, for example, in the Snails and kindred forms, 

 both terrestrial and marine, it is on'y in those 

 parts of the body that are protrusible from the 

 testaceous covering that the tegument exhibits 

 this decided muscularity, the mantle lining the 

 shell being constantly thin and membranous. 



But the most strongly developed part of the 

 muscular covering of a gasteropod is the broad 

 fleshy disc attached to the ventral surface of 

 the body which constitutes the apparatus of 

 locomotion, and gives the name conferred by 

 zoologists upon the entire class. This disc, or 

 foot, as it is likewise called, is entirely made 

 up of contractile fibres, disposed in various 

 directions, so as to confer all the capabilities 

 of movement necessary for securing progres- 

 sion along the plane surfaces over which these 

 sluggish animals are destined to crawl. 



Having, as yet, no internal skeleton deve- 

 loped, and being equally destitute of any thing 

 like an external articulated frame-work, it must 

 be evident that if creatures of this description 

 are to be provided with organs requiring to be 

 moved by subordinate sets of muscles, seeing 

 that there is no firm point of attachment to be 

 found, as is the case among Insects or the 

 Vertebrata, recourse must be had to a new 

 plan, and accordingly few more remarkable 

 deviations from what is generally met with in 

 other animals can be pointed out than we meet 

 with in the extensive class under consideration. 

 The parts of the mouth, the tentacles, the eyes, 

 and those parts of the male generative system 

 needed for copulation, are in many instances 

 so constructed that they may, when not in use, 

 be completely retracted into the general cavity 

 of the body and packed up amongst the viscera 

 by means of a mechanism quite peculiar, and 

 of which a particular account is elsewhere 

 given. (See GASTLKOPODA.) 



