340 



THE CRAYFISH AND THE ARTHROPODA 



pedes, or "thousand-legged worms." These are famihar animals 

 in all temperate and tropical countries, living in damp places 

 under stones and timbers and in the upper layers of the soil. 



There is a head region, but the worm- 

 like shape of the body and the many 

 appendages are primitive features. In 

 the subdivision of the myriopods that 

 includes the millipedes, there are two 

 pairs of legs to each true somite. 

 Respiration is carried on by a tracheal 

 system {cf. p. 355). Hence, when all 

 the facts of their structure and de- 

 velopment are considered, it seems 

 that they are the type of arthropod 

 most closely related to the insects, 

 although not specialized like the mem- 

 bers of this class by a division of the 

 body into thorax and abdomen, the 

 loss of abdominal appendages, and 

 development of the two pairs of wings 

 that are characteristic of most insects. 

 Class Ins seta. — The Class Inseda 

 arc to-day the dominant forms of life 

 upon the land so far as number of 

 species is concerned, although they are 

 exceeded in size of individuals by 

 most other types of terrestrial animals. 

 With respect to their intelligence, 

 they may also be compared more 

 favorably with the Chordata than 

 may any other class of animals, 

 despite their specialization along the 

 lines of inherited and unmodifiable 

 reflexes in contrast with the capacity 

 for acquiring individual habit reflexes that characterizes the 

 higher chordates. 



Class Arachnida. — The most familiar examples of the Class 

 Arachnida are the many species of spiders (Fig. 167); but the 

 scorpions, the mites, and the ticks are all members of this class, 

 which is, therefore, highly diversified. All these arachnids are 



Fjg. 166. — The house centi- 

 pede, Scutigera forceps. 



(From Farmers' Bulletin, No. 627, 

 U. S. Dept. Agr.) 



