MORPHOLOGY OF HIGHER TYPES 



135 



whose outer ends project slightly from the body. These sctse are 

 pivoted in the body wall and their inner ends are moved by the contrac- 

 tions of muscles (Fig. 100). The setae play an important part in the 

 locomotion of the earthworm because after the body is extended they 

 catch on the soil and hold fast until the contraction of the longitudinal 

 muscles shortens the body and brings up the rear. Since they are pivoted 

 and are provided with muscles their angle with the surface can be changed. 

 With the angle of the setae changed the contraction and relaxation of 

 the circular and longitudinal muscles move the worm in the opposite 

 direction 



In the Arthropoda (including shrimps, crayfish, crabs, insects, and 

 spiders) the locomotor organs likewise consist of sets of levers moved 

 by the contraction of muscles regardless of whether the form of locomo- 

 tion be swimming, creeping, walking, or flying. The hard parts are 



TRUNK, 



Fig. 102. — Regions of the vertebrate skeleton (cat). {From Jayne's Mammalian Anatomy.) 



outside and the muscles inside, as shown in (Fig. 101, A). This relation 

 of levers and muscles is reversed in the vertebrate leg (Fig. 101, B), 

 in which the bones are beneath the muscles. 



Skeleton of Vertebrates. — The vertebrate skeleton largely deter- 

 mines the form of the bod}' and serves as attachment for muscles and 

 other tissues. It is composed of bones and cartilages united partly by 

 ligaments, is covered by the soft parts of the body and is supplied with 

 blood vessels and nerves. It may conveniently be divided into regions 

 as indicated in Fig. 102. On more fundamental anatomical grounds 

 it is also subdivided into the axial and the appendicular skeleton. 



Axial Skeleton. The axial skeleton (Fig. 103) is made up of 

 the skull, hyoid apparatus, vertebral colmnn, ribs, and sternum. The 

 skull furnishes a case for the brain, capsules for the organs of hearing and 

 smell, and orbits for the eyes. It also includes the bones of the jaws. 



