INTRODUCTION TO THE METAZOA 77 



adults in which locomotion results from ciliary action. In the 

 Metazoa, movements and locomotion more frequently result 

 from the activity of contractile fibers which represent different 

 degrees of specialization or differentiation of muscle cells. In 

 some instances, there are scattered contractile elements, but more 

 commonly the contractile elements are associated to form 

 continuous sheets or bundles of muscles. 



The dermomuscular sac of flatworms represents a definite 

 stage in the development of a locomotor system. Alternate 

 contraction and relaxation of the longitudinally and circularly 

 directed fibers in such a sac produce successive shortenings and 

 elongations of the body. As a consequence locomotion results. 

 When hard skeletal parts make their appearance, groups of mus- 

 cle cells become attached to these hard parts and with fixed points 

 of attachment perform more effective movements. Even the 

 setae of an earthworm serve for muscle attachment and aid in 

 locomotion, thereby suggesting an extremely early stage in the 

 differentiation of a locomotor skeleton. The exoskeleton of the 

 arthropods is the most highly organized locomotor skeleton 

 found in the invertebrates. 



Nervous and Sensory System, — Histological elements differ- 

 entiated for transmission of nervous impulses assume sundry 

 forms of organization in the metazoan body. The hydroid or 

 polyp stage of the coelenterates is usually provided with a 

 scattered or diffuse arrangement of nerve cells, but in all of the 

 Metazoa above the coelenterates the nerve cells are grouped to 

 form definite tracts of nerve tissue. In these tracts occur not 

 only the cell bodies of the neurons and cytoplasmic fibers with 

 their specializations as already described but also supporting 

 elements of which the neuroglia cells are the most important. 

 Usually, there is a tendency for the cell bodies of the neurons to 

 become massed in certain parts of the nerve tracts. These 

 regions are designated as ganglia. 



The nervous systems of medusae are interesting in that they 

 exhibit a series of steps toward the centralization of the nervous 

 system. In the hydromedusae, a nerve ring without any differ- 

 entiation of ganglia parallels the margin of the bell. In the 

 scyphomedusae, the nerve ring has become more specialized and 

 shows localization of the cell bodies to form ganglia. 



Among metameric animals, there is a tendency for ganglion 

 formation in each somite, but this primitive scheme is in many 



