444 
alternate leg on one side supports the animal, 
and urges the centre of gravity forwards, 
whilst the corresponding leg on the other side 
is raised and advanced to take a new position 
on the plane of motion. Such, at least, seems 
to be the process employed, but there is very 
great difficulty in ascertaining their motions 
with perfect accuracy ; we therefore give the 
above in part practically, and in part hypothe- 
tically. The Myriapods move with considerable 
agility ; usually run, and if disturbed, continue 
that pace for a great length of time. They have 
the power of climbing with facility the per- 
pendicular surfaces of trees, walls, &c. 
Arachnida.—The Arachnida are furnished 
with four pairs of legs, which render their mode 
of progression more complex, and more diffi- 
cult of observation, than that of the Decapods.* 
Their coxe are articulated to the base of the 
cephalo-thorax by cotyloid joints arranged in 
an elliptical form, and directed horizontally 
outwards ; all the remaining articulations of the 
legs are ginglymoid, which is the best mode of 
articulation for the horizontal movements of the 
leg whilst urging the body forwards ; the tarsi, 
which are composed of a variable number of 
segments, terminate by a double or single hook, 
which affords to this tribe of animals the means 
of ascending vertically, whenever any surfaces 
resent minute irregularities adapted for pre- 
ension ; the legs, projecting from the cephalo- 
thorax horizontally, increase the base of support. 
The centre of gravity is that of an ellipse. 
The Arachnida cannot ascend vertically on 
glass, but are enabled to walk in an inverted 
position on ceilings or rough surfaces without 
the assistance of their web. 
The organs of motion in spiders, though 
nearly constant in number, differ exceedingly in 
length; the general principle is, however, the same. 
After long and repeated observation, I disco- 
vered the order in which the eight legs of these 
animals are putin motion. If we first attend 
to the manner in which the legs are moved 
on either side singly, they will be found to 
move first the fore leg, 
then the fourth, then 
the third, and lastly, the 
second leg; that is, in 
the order 1, 4, 3,2. On 
observing the motion of 
the legs on both sides of 
theanimalsimultaneous!y, g 
they are found to move the 
first right leg, then the 
fourth left; then the first 
left and the fourth right ; 
then the third right and 
the second left ; and lastly, 
the third left and the se- 4 4 
cond right (fig. 238). 
Of these, the first two sets are moved conse- 
cutively, like those of a quadruped, 1’, 4, 1, 4’; 
the last two in pairs simultaneously, that is, 
3’, 2, then 3, 2’; and whilst the legs of one 
side of the animal are moving consecutively 
Fig. 238. 
* The female is furnished with an additional 
pair, to enable her to carry her eggs. 
MOTION. 
in the order 1, 4, 3, 2, the legs of the ot 
side are moving in pairs in the inverted o 
4’, 1’, 2’, 3’. In descending vertically by m 
of a newly-spun thread, they hang by one o 
the hind legs; on ascending the same thread 
they employ three legs; the two first on one 
side, and the first or soon on the other; in 
running, the second pair of legs is placed im 
advanne of the first, ben choos Gf the! third be. 
fore the second, thus giving the feet a greater 
range of space at each step. The fourth pair 
or hind legs, are directed beck ward nearly pa- 
rallel to the line on which the animal is 
moving ; they seem to be chiefly used to support 
the posterior segments of the abdomen, but 
also exercise a limited action in propelling the 
body forwards. Four legs support the body 
almost, but not quite, simultaneously, as stated 
by Professor Miiller, whilst the other four 
are raised. The progression of the spider is 
usually rapid ; it can run upon its web with 
great facility ; it can leap many times its own 
length in chace of prey; it can float during a 
limited period on water; and the facility with 
which it can spin and throw a OSS 
cavities from one fixed point to another, at a 
considerable distance, endows it with a mode 
of transit across spaces which is denied to many 
other animals. a 
Decapoda.—The modes of progression 
ployed by the Deepa are both various 
singular. Organized either to swim in 1 
or seas, to walk and run on the dry la 
at the bottom of water, both fresh and 
they are furnished with organs of locomotion 
suitable for these different purposes. The five 
pairs of legs articulated at the base of 
cephalo-thorax have the whole of the ji 
articulated, and directed to move on sol; 
either laterally or directly backwards. The 
front legs are generally the most massive and 
powerful, throwing the centre of gravity for- 
wards nearly between the axes of ther articu- 
lations. 
The Brachyurous Decapods, as the Cancer 
Maia, §c., present either a quadrilateral or 
pyriform figure. They are generally destitute 
of the great elongations of the abdominal si 
ments and expansion of tail into fins for swim. 
ming, which we find in the Macrourous De. 
capods. The consolidated carapace of the di 
capod tribe deprives them of lateral flexibility 
in the thoracic section of the trunk. The lai 
species of Decapods, such as the Cancer cursu 
or land crabs, are capable of running with : 
velocity, that a man on horseback has difficul 
in keeping up with them. From their spee 
they were called by the more ancient natu 
ralists equi. In many species, such as 
Inachus thoracicus, the Leptopus longipes, at 
the Leptopodia sagittaria, the legs are grea 
elongated, and consequently exercise a locon 
tive office resembling that of the tipula among 
insects, differing however from it in the direc 
tion of the articulations, by which the pr 
gression of these different classes is 
Thus in the Leptopus longipes (fig. 
action resulting from the flexion anc 
tension of the legs in gf, g’ f’ will 
” 
acro 
239), t 
