CHAPTER 31 



OJI 



Skeletal Systems 

 and Movement 



HE advantages of having a skeleton were 

 made evident by our study of the animals 

 that do not have one. The jellyfish can float 

 and swim in the sea, but on land it is help- 

 less. All vertebrates, such as fishes, birds, and 

 man, have skeletons. Imagine man without 

 a skeleton— he could not build the sky- 

 scrapers, the bridges, or the jets that enable 

 him to fly faster than the birds— he would be 

 a mass of tissue with little form and 

 strength. The skeletal structures determine 

 the form of an animal. They serve three pur- 

 poses: (1) to provide a support or frame- 

 work for the softer parts of the body, (2) 

 to protect the soft parts, and (3) to supply 

 a firm surface for muscle attachment. 



Types of skeletal structure 



In general we may recognize two types of 

 skeletons: (I) exoskeletons that are built 

 up on the outside of the body and to whose 

 inner surface muscles are attached and (2) 

 endoskcletons that are built up inside of 

 the body, are surrounded by soft tissues, 

 and have muscles attached to their outer 

 surface. Besides these, there are many skele- 

 tal structures, especially among the lower 

 invertebrates, that furnish support and pro- 

 tection but consist of more or less isolated 

 parts that are not joined together into a 

 unified whole. 



Motion and locomotion do not depend 

 much on the presence of skeletal structures 

 in the lower invertebrates; but in the rest of 

 the animal kingdom, movements of ani- 

 mals require either an exoskeleton or an en- 

 doskeleton for the attachment of muscles. 



498 



Protection and support 



Protection and support are secured among 

 the lower invertebrates in various ways. 

 Protozoa may secrete a shell, such as that 

 of Arcella (Fig. 17), or they may build up a 

 shell of tiny grains of sand held together by 

 a secretion, such as that of Difflugia (Fig. 

 17). The protection these shells provide en- 



