CHAPTER VI. 



Jointed Animals, — continued. 



Centipedes, Millipedes, Scorpions, and Spiders, — Classes 

 Chilopoda, Diplopoda, Arachnida, etc. 



Characters of According to modern views, centipedes are regarded as near allies 

 Centipedes. f insects, the chief differences between the two groups being that 

 whereas in the latter there are only three pairs of jaws attached to the lower 

 surface of the head, in the former four pairs of appendages are modified to act 

 as masticating organs. Moreover, the body of an insect is sharply divided 

 into an anterior portion, or thorax, bearing three pairs of walking legs, and a 

 posterior half, or abdomen, which in the adult at least is not provided with 

 locomotor limbs, but the body of a centipede is composed of a large and varying 

 number of segments, substantially alike in structure, and each bearing a single 

 pair of legs. The number of segments varies from fifteen to considerably over 

 one hundred, yet no matter how many pairs of legs there may be — whether it be 

 fifteen or one hundred and twenty-one — their number is invariably odd. 



The head bears a pair of elongate antenna? in front, and often eyes arranged 

 in two clusters at the sides. On its lower surface may be seen the four pairs 

 of jaws. The first pair, or mandibles, are two-jointed and have a biting edge ; 

 the second pair, or maxillae, are soft, leaf-like, and united together in the middle 

 line, each consisting of an outer jointed and an inner unjointed branch. The third 

 pair, known as the first maxillipedes, are composed of four or five segments, and 

 much resemble one of the walking limbs, being tipped with a claw. The fourth 

 pair, or second maxillipedes, are large, powerful, and project forwards below the 

 rest, so as more or less to conceal them from view. Their basal segments are 

 usually fused to form a massive coxal plate, while the rest of the jaw consists of 

 four segments, the terminal one being a long fang with a minute aperture at the 

 tip, through which exudes poison secreted hy a gland lodged inside the appendage. 

 These two pairs of maxillipedes do not strictly belong to the head, since the dorsal 

 elements of the segments that bear them are either distinct, or are united with 

 the tergal plate of the following segment to constitute a- massive basilar plate. 



The rest of the body is composed of a varying number of segments, each 

 consisting externally of a dorsal plate or tergum, and a ventral plate or sternum, 

 connected laterally by a softer pleural membrane, to which the legs are articulated. 

 These latter are usually short, composed of six or seven segments, and each is tipped 

 with a single claw, and often furnished in addition with spines. The last pair are 

 generally longer and stronger than the rest, and sometimes considerably modified 



