444 



MOTION. 



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. 



Aruchnida. 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 coxae 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 

 present minute irregularities adapted for pre- 

 hension ; 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 put in 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 

 the animal simultaneously, 

 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- 

 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 



" The female is furnished with an additional 

 pair, to enable her to carry her eggs. 



Fig. 238. 



in the order 1, 4, 3, 2, the legs of the other 

 side are moving in pairs in the inverted order 

 4', 1', 2', 3'. In descending vertically by means 

 of a newly-spun thread, they hang by one of 

 the hind legs ; on ascending the same thread 

 they employ three legs ; the two first on one 

 side, and the first or second on the other ; in 

 running, the second pair of le^s is placed in 

 advance of the first, then those of 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 backwards 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 Muller, 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 cord across 

 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. 



Decapoda. The modes of progression em- 

 ployed by the Decapods are both various and 

 singular. Organized either to swim in rivers 

 or seas, to walk and run on the dry land or 

 at the bottom of water, both fresh and salt, 

 they are furnished with organs of locomotion 

 suitable for these different purposes. The five 

 pairs of legs articulated at the base of the 

 cephalo-thorax have the whole of the joints 

 articulated, and directed to move on solids 

 either laterally or directly backwards. The 

 front legs are generally the most massive and 

 powerful, throwing the centre of gravity for- 

 %vards nearly between the axes of their articu- 

 lations. 



The Brachyurous Decapods, as the Cancer, 

 Maia, <$<., present either a quadrilateral or a 

 pyriform figure. They are generally destitute 

 of the great elongations of the abdominal seg- 

 ments and expansion of tail into fins for swim- 

 ming, which we find in the Macrourous De- 

 capods. The consolidated carapace of the de- 

 capod tribe deprives them of lateral flexibility 

 in the thoracic section of the trunk. The land 

 species of Decapods, such as the Cancer cursus, 

 or land crabs, are capable of running with such 

 velocity, that a man on horseback has difficulty 

 in keeping up with them. From their speed 

 they were called by the more ancient natu- 

 ralists c(jui. In many species, such as the 

 Inachus thoracicus, the Leptopus lojigipes, and 

 the Leptopodia sagittaria, the legs are greatly 

 elongated, and consequently exercise a locomo- 

 tive office resembling that of the tipula amongst 

 insects, differing however from it in the direc- 

 tion of the articulations, by which the pro- 

 gression of these different classes is reversed. 

 Thus in the Leptopus longipes (Jig- 239), the 

 action resulting from the flexion and ex- 

 tension of the legs in g J\ g' J ' will propel 



