GENERAL CHARACTERS OF ARTHROPODS. 285 



CHAPTER VII. 



BRANCH VII. AETHROPODA (CRUSTACEANS AND 

 INSECTS). 



General Characters of Arthropods. To this group be- 

 long those Articulates which have jointed appendages, i. e., 

 antennae, jaws, maxillae (or accessory jaws), palpi, and legs 

 arranged in pairs, the two halves of the body thus being 

 more markedly symmetrical than in the lower animals. The 

 skin is usually hardened by the deposition of salts, carbon- 

 ate and phosphate of lime, and of a peculiar organic sub- 

 stance, called chitine. The segments (somites or arthro- 

 meres) composing the body are usually limited in num- 

 ber twenty in the Crustaceans and eighteen in the insects 

 while each arthromere is primarily divided into an upper 

 (tergum), lower (sternum), and lateral portion (pleurum). 

 These divisions, however, cannot be traced in the head either 

 of Crustaceans'or insects. Moreover the head is well marked, 

 with one or two pairs of feelers or antennae, and from 

 two to four pairs of biting mouth-parts or jaws, and two 

 compound eyes ; besides the compound eyes there are simple 

 eyes in the insects. The germ is three-layered, and there is 

 usually a well-marked metamorphosis. The Arthropoda 

 are nearest related to the worms, certain Annelides, with 

 their soft- jointed appendages (tentacles as well as lateral 

 cirri) and well-marked head anticipating or foreshadowing 

 the Arthropods. On the other hand, certain low parasitic 

 Arthropods, as Linguatula, have been mistaken for genuine 

 parasitic worms. So close are the affinities of the Vermes 

 and Arthropods that they were by Cuvier united as a Branch 

 Articulata, and while the Annelides and Arthropods may 

 have had a common parentage, the recent progress in our 

 knowledge of the worms, has led naturalists to discard the 



