128 BIOLOGY: GENERAL AND MEDICAL 



vanced segments as the remainder are drawn after them. 



Among the arthropods the function of locomotion 

 becomes highly specialized by the development of jointed 

 appendages controlled by muscles attached to all or 

 certain of the segments. 



Among the vertebrates the same general plan of 

 having the motor organs spring from certain of the 

 body segments is preserved, though they undergo great 

 modification in their specialization into fins, wings, legs, 

 arms, flippers, etc., with complicated muscular and 

 other adjustments. 



Frequent allusion has been made to muscle cells and 

 muscles so that it becomes necessary to say a few words 

 about these as important adjuncts to the motor apparatus. 



In the tentacles of certain hydras, in the higher ccelen- 

 terates, and in all of the higher animals there are certain 

 mesenchymal cells that specialize in contractility. 

 These are known as muscle cells. They are of elongate 

 shape, but are capable of manifesting their contractile 

 power by shortening and thus making traction upon the 

 structures to which they are attached. Primarily of 

 this spindle shape and appearing singly, they are asso- 

 ciated in groups and bundles in the higher animals where 

 their combined action is very effective as sources of 

 movement. Eventually they appear as elongate multi- 

 nucleated, transversely striated fibrils, singly or in bun- 

 dles the voluntary muscles which are the source of 

 the extensive and powerful movements of the higher 

 animals. 



The Circulatory System. So soon as cell combinations 

 become so large or so differentiated as to make it im- 

 possible for each cell to exist under conditions common 

 to all the cells, it becomes desirable that some special 

 means be provided by which the less advantageously 

 situated cells may be provided with nourishment and 

 have their effete products removed. 



In the most simple cell colonies, such as Microgromia, 

 Carchesium, Epistylus, and Volvox, the cells, though 



