CHAPTER VII. 
BRANCH VIIL—ARTHROPODA (CRUSTACEANS AND 
INSECTS). 
General Characters of Arthropods.—To this group be- 
long those Articulates which have jointed appendages, 7. e., 
antenne, jaws, maxille (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 
(tergura), 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 antenne, 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 
