GENERAL ZOOLOGY 



effects of one of the most potent poisons known to man, hydrocyanic acid gas. 

 These are examples of the general adaptability of living organisms to en- 

 vironmental changes and may be regarded as the result of a process of 

 artificial selection. According to this interpretation, certain individuals, by 

 the chance operation of random mutations, are capable of withstanding 

 ordinarily lethal concentrations of the poison. Continued subjection to the 

 insecticide rigorously "selects" for survival those individuals which are most 

 resistant, until eventually the breeding population is composed entirely, or 

 preponderantly, of resistant stocks. 



THE CLASSES CHILOPODA AND DIPLOPODA 



Members of these two arthropod classes show certain superficial similarities: 

 they are tracheate animals with mandibulate mouth parts, with long, slender 

 bodies composed of a large but indefinite number of segments, and with 

 numerous paired, jointed appendages. These similarities have led to their 

 classification within a single group, the class Myriapoda, in older systems. 

 However, closer study reveals many significant differences between centipedes 

 and millipedes which indicate a more remote relationship. 



Centipedes are somewhat flattened dorsoventrally; the body is composed 

 of a definite head — bearing antennae, ocelli, and the mouth parts — and a many- 

 segmented trunk (Fig. 15.38). Each of the trunk segments, except the last 

 two, bears a pair of walking legs; those of the first segment are modified into 

 conspicuous poison fangs, sometimes referred to as maxillipeds. The gonads 

 are dorsal to the gut, and, as in insects, the genital openings are at the 

 posterior end of the body. Centipedes are predatory carnivores, feeding on 

 small animals captured and killed with the aid of the poison fangs. 



Millipedes are cylindrical and worm-like. As in centipedes, a head is 

 present, bearing antennae and groups of ocelli. The trunk, however, may 

 be divided into four anterior thoracic segments and a large number of 

 abdominal segments. Those of the abdominal region are fused in pairs, so 

 that each apparent division seems to bear two pairs of walking legs (Fig. 

 15.38). The gonads lie ventral to the gut, and the genital openings are 

 anterior, on one of the thoracic segments. Millipedes are retiring, her- 

 bivorous animals which when disturbed usually roll up into a tight spiral 

 and emit a disagreeable odor from segmental scent glands. 



Diplopods, and some chilopods, are characterized by anamorphic growth: 

 the young individual does not have the full adult number of somites, but at 

 each molt as it grows to maturity new segments are added in a posterior zone 

 of growth, just anterior to the segment bearing the anus. As we have seen, 

 this type of growth is characteristic of annelids, as well as of the most 

 primitive insects (Protura). Other features reminiscent of annelids are the 

 long, worm-like body, the relatively undifferentiated nature of the somites, 

 and the presence of paired appendages on most of the segments. More ad- 

 vanced characteristics include the development of a definite head with specific 



478 



