100 
ZOOLOGY, 
was like that of a king-crab, and after a number of moults 
acquired its thoracic segments, there being in most of them 
a well-marked metamorphosis. The Trilobites occur in 
the oldest fossiliferous rocks. Fig. 126 is an attempt by 
Mr. C. D. Walcott to represent a restoration of a cross-sec- 
tion of a trilobite, showing the relations of the feet and 
gills to the body; the gills being spirally twisted filaments 
growing from the base of the legs. 
Iksecta akd other air-breathing Arthropoda. 
General Characters of Insects. — In the insects the head 
is separated from the rest of the body, which is divided 
into three regions, the head, thorax, and hind-body (ab- 
domen); hence the name insect, from insectum, cut into 
or divided. Insects breathe by internal air-tubes which 
open through breathing-holes (spiracles) in the sides of the 
body. The six-footed insects also have two pairs of wings. 
The number of body-segments in winged insects is seven- 
teen or eighteen — i,e,, four in the head, three in the thorax, 
and ten or eleven in the hind-body. In spiders and mites 
there are usually but two segments in the head, four in the 
thorax, and a varying number (not more than twelve) in 
the abdomen; in Myriopods the number of segments varies 
greatly — i.e,, from ten to two hundred. The appendages 
of the body are jointed. 
Of the winged insects there are two types: first, those in 
which the jaws and maxillae are free, adapted for biting, 
as in the locust or grasshopper; and, second, those in which 
the jaws and maxillae are more or less modified to suck or 
lap up liquid food, as in the butterfl}?, bee, and bng. 
Nearly all insects undergo a metamorphosis, the young 
being called a larva (caterjoillar, grub, maggot); the larva 
transforms into ?i pupa (chrysalis), and the pupa into the 
adult (imago). 
In order to obtain a knowledge of entomology;, the be- 
