100 ZOOLOGY. 



was like that of a king-crab, and after a number of monlts 

 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. 



Insecta and 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 butterfly, bee, and bug. 



Nearly all insects undergo a metamorphosis, the young 

 being called a larva (caterpillar, grub, maggot); the larva 

 transforms into a pupa (chrysalis), and the pupa into the 

 adult (imago). 



In order to obtain a knowledge of entomology, the be- 



