GENERAL CHARACTERS OF INSECTS. 307 
through the heart, and the finer arteries traced into the large claws 
and legs. In the crab, the entire upper side of the carapace may be 
removed by the point of a knife. The smaller Crustacea, especially 
the water-fleas, may be examined alive under the microscope as trans- 
parent objects. In the larger forms the stomach may be laid open by 
the scissors in order to study its complicated structure. The eyes of 
the lobster should be hardened in alcohol and fine sections made for 
the microscope. This is an operation requiring much care and expe- 
rience. Experts in embryology have sliced the eggs of certain Crusta- 
cea and studied their embryology with great success. 
Tur AIR-BREATHING ARTHROPODA (Centipedes, Spiders. 
Insects). 
General Characters of Insects.—While in the worms 
there is no grouping of the segments into regions, we have 
seen that in most Crustacea there are two assemblages of 
segments—4t. e., a head-thorax and abdomen. In the insects 
there is a step higher in the scale of life, a head is separated 
from the rest of the body, which is divided into three 
regions, the head, thorax, and hind-body (abdomen). More- 
over, the insects differ from the Crustacea in breathing by 
internal air-tubes which open through breathing-holes 
(spiracles) in the sides of the body. The six-fogted insects 
also have wings, and their presence is correlated with a 
differentiation or subdivision of the two hinder segments 
of the thorax into numerous pieces. 
The number of body-segments in winged insects is seven- 
teen or eighteen—7. ¢., 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, and perform four different func- 
tions—/. e., the antenne are sensorial organs, the jaws and 
maxille are for seizing and chewing or sucking food ; the 
