THE NATURAL HISTORY SURVEY. i6/ 



two regions passing between the sixth and seventh pairs of 

 appendages. The segments of the cephalothorax are usually 

 coalescent, those of the abdomen being either free or not. The 

 antennae are absent. Two sub-classes are recognized under 

 this class, the MEROSTOMATA being aquatic forms which are for 

 the most part extinct, but are represented by the living Limulus; 

 and the ARACHNIDA or spiders, scorpions and their allies, in 

 which the primitive branchias have been modified for aerial 

 respiration. Because of their terrestrial habits the arachnids 

 are rarely preserved as fossils. 



The TRACHEATA are divided into two classes, the MYRIOPODA 

 or thousand -legged worms and centipedes, and the HEXAPODA 

 or insects. The body of the myriopods is worm-like, and con- 

 sists of a distinct head, followed posteriorly by numerous similar 

 segments, each of which bears one or two pairs of appendages. 

 The body of the mature insect, HEXAPODA, has the three regions, 

 head, thorax and abdomen, fully differentiated. The append- 

 ages of the head are specialized sensory organs, the antennae, 

 and the mouth parts; the thorax bears the organs of locomotion, 

 three pairs of legs and usually two pairs of wings. 



In the process of growth all arthropods undergo a series of 

 moults. The exoskeleton with which all are furnished, is 

 capable of but limited expansion, so that if these creatures are 

 to increase in size it is necessary for them to shed this covering 

 periodically. With each successive moult the new exoskeleton 

 which is secreted is somewhat larger than the preceding one. 

 During the stages of immaturity the creatures assume after each 

 moult, new characters which successively approach more and 

 more nearly those of the mature individual, but after the adult 

 characteristics of body and appendages have been acquired, the 

 succeeding changes at the moulting periods are only an increase 

 in size until the maximum dimensions have been attained. 

 Among those aquatic arthropods, the crustaceans and some 

 Acerata, whose habitat is most favorably situated for preserva- 

 tion in a fossil condition, the abandoned exoskeleton is frequently 

 firm and hard and fully capable of being buried in the sand or 

 mud to become fossilized at some later period. On account of 

 their habit of moulting, each one of these creatures has an 

 opportunity to leave a fossil record of itself as many times as the 

 number of its moulting periods. This may account for the great 

 abundance of fossil trilobites in some formations. The large 

 number of fossil remains, in some cases, may not really mean that 



