540 
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-thorax 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 attan 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 sha 
resemble the larve of aquatic Insects. Tn 
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 Brauchyura, 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 Crab on first leaving the egg is 
almost in the conditiou of a long-tailed Shrimp, 
MUSCULAR SYSTEM. 
and the locomotive limbs scarcely to be 
nised as being worthy of such a ttle, being not 
only rudimentary in their size but exclusive 
adapted for swimming; and it is only 
several times casting its shell and p 
through distinct sere, ee of form, 
muscles of the less attain the preponde: 
over those of the trunk and become st 
enough for progression on land. a 
The fourth grand division of the ani 
kingdom, comprising the MOLLUSCA © 
Cuvier, is characterized by the he ap vi 
dition of the nervous ganglia, which, throu 
out the extensive series of creatures construct 
according to this type, are distributed witl 
any symmetrical arrangement in different pat 
of the body, whence the Mollusca have 
nained by Professor Grant CycLo-GaNGLIAT 
and more recently by Professor Owen HrrEr 
GaNGLtaTA, the latter term being, as we ¢ 
ceive, the preferable of the two. In the Me 
lusea the general outline of the body parti 
pates, more or less, in the want of syinmel 
that is so conspicuous in the disposition ¢ 
ganglia composing the nervous system, and # 
muscular apparatus does not exhibit that 
cision and regularity which is visible among 
the Articulata. There is no longer, in f 
any trame-work, but when a shell is presi 
as, for exainple, in the Snails and kindred fort 
both terrestrial and marine, it is on'y in th 
parts of the body that are protrusible from 1 
testaceous covering that the tegument exhi 
this decided muscularity, the mantle lining 
shell being constantly thin and membranou: 
But the most strongly developed part of 
muscular covering of a gasteropod is the br 
fleshy disc attached to the ventral surface 
the body which constitutes the appa 
locomotion, and gives the name conferred 
zoologists upon the entire class. This dise, 
foot, as it 1s likewise called, is entirely m 
up of contractile fibres, disposed in va 
directions, so as to confer all the capabi 
of movement necessary for securi Og 
sion along the plane surfaces over which t 
sluggish animals are destined to crawl. — 
Having, as yet, no internal skeleton ¢ 
loped, and being equally destitute of anyt 
like an external articulated frame-work, it 
be evident that if creatures of this de 
are to be provided with organs requiring 
moved by subordinate sets of mu 4 
that there is no firm point of attachment 
found, as is the case among Insects 
Vertebrata, recourse must be had to 
plan, and accordingly few more a 
deviations from what is generally met 
other animals can be pointed out than W 
with in the extensive class under conside} 
The parts of the mouth, the tentacles, th 
and those parts of the male generative § 
needed for copulation, are in many inst 
so constructed that they may, when not! 
be completely retracted into the general ¢ 
of the body and packed up amongst the v 
by means of a mechanism quite peculiar 
of which a particular account is 
given. (See GasTiropopa.). 
