CHAPTER VI 

 JOINTED ANIMALS Continued 



CENTIPEDES, MILLIPEDES, SCORPIONS, AND SPIDERS CLASSES 

 Chilopoda, Diplopoda, Arachnida, etc. 



ACCORDING to modern views, centipedes are regarded as near allies 

 of insects, the chief differences between the two groups being that 

 Centipedes 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 loco- 

 motor 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 antennae 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 v 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 forward 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 by 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 con- 

 sisting externally of a dorsal plate or tergum, and a ventral plate or sternum, con- 

 nected 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 in 

 structure. Breathing is effected by tracheal tubes, which open by means of stig- 

 mata, placed almost always upon the pleural membrane of the segments. 

 (3170) 



