2 INVERTEBRATE ZOOLOGY 
which is composed largely of a very hard and resistant substance 
called chitin, and serves the double purpose of a protection for 
the internal soft parts and a surface for the attachment of 
muscles. It is, in fact, the skeleton of the animal, and is called 
an exoskeleton, in contradistinction to an internal supporting 
structure which would be called an endoskeleton. All inverte- 
brate animals, except some of the lowest, are provided with a 
cuticular exoskeleton, but it is only the arthropods in which it 
is composed largely of chitin. In fact, the possession of such 
a hard and resistant external covering is one of the reasons why 
insects have so successfully maintained themselves in the uni- 
versal struggle for existence. 
Observe that the body of the animal is composed of a number 
of serially arranged segments. These are called somites or meta- 
meres, and the segmented type of structure presented by the 
insect body is called a metameric type of structure. Observe 
that the body is sharply divided into three divisions — the head, 
thorax, and abdomen. 
The head is unsegmented and bears on its anterior and dorsal 
surface a pair of long, jointed feelers or antenne, which are impor- 
tant sense-organs, a pair of large compound eyes, and three small, 
dot-like eyes, called ocelli, which it may be necessary to look for 
with a hand lens; on its ventral side are the mouth-parts, the 
organs which taste, grasp, and masticate the food. Examine 
these mouth-parts carefully with a hand lens ; notice that there 
is a short overhanging upper lip, beneath which is a pair of 
powerful jaws having a lateral or side position instead of a 
dorso-ventral one like the jaws of vertebrates. Beneath the 
jaws are two other pairs of mouth-parts, the maxille and the 
under lip, which, however, will not be studied at present ; notice 
the two pairs of elongated and segmented palps, which are 
probably organs of taste. 
The thorax is composed of three somites or metameres, which 
are called, respectively, the pro-, meso-, and metathorax. Each 
