CHAPTER VII. 



BEANCH VII. ARTHROPODA (CRUSTACEANS 



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 

 (terffura), lower (sternum), and lateralr portion (pleurum), 

 ever, cannot be traced in the head either 

 ?cts. 'Moreover the head is well marked, 

 \\nii one or two pairs of feelers or antennas, 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 Veraies 

 and Arthropods that they were by Cuvier united as a Branch 

 Articulatd, and while the Annelides and Arthropods may 

 have had a common parentage, the recent progress in our 



