336 



THE MAINTENANCE OF THE INDIVIDUAL 



madreporite by means of cilia which send it along the stone canal. 

 The latter structure leads straight down to the circumoral ring canal. 

 Five radial canals branch from this and extend down the five arms 

 sending off smaller branches which end in the tube-feet lying along the 

 ambulacral grooves. The proximal end of each foot has a muscular 



ws/ s :Q 



Vertical section through an arm of a starfish : b, ampulla ; d. water canal 

 opening at madreporite plate, sl\ i, radial water tube; m, mouth; //, tube feet; 

 py, digestive gland ; sic, stomach ; a, anus ; v, ring canal ; n, nerve ring. 



bulb, the ampulla, which is capable of contracting, thus forcing the 

 ambulacral fluid into the tube-foot. When the sucking disks at the 

 free end of the distended tube-feet become attached to an object, the 

 muscles of these tubular organs contract, forcing the water back into 

 the ampullae, and the animal through its grip is enabled to move 

 forward. 



Muscles and Muscular Systems 



Great differentiation of muscles is invariably related to a well- 

 developed skeletal system. In two large diverse groups of animals, 

 the arthropods, with a chitinous exoskeleton, and the vertebrates, 

 with a calcareous endoskeleton, individual muscles rather than 

 muscle layers have been developed. Examples of exoskeletal muscles 

 are the colorless, transparent, or yellowish-white muscles typical of 

 the insects. Although soft and almost gelatinous in appearance, these 

 muscles which are usually striated are very efficient, as may be seen 

 in the common house fly whose wings beat over 300 strokes per 

 second. Among vertebrates there are found smooth or involuntary 

 muscles, skeletal or striated muscles, and heart muscles. 



While the muscles of a frog and those of a man may be homologous, 

 that is, comparable embryologically and morphologically, it does 

 not necessarily follow that they are analogous, that is, alike in 



