WORMS 317 



On the lower (ventral) side of the "arms," or "rays," 

 the hard coverings are turned inward, forming five 

 grooves. Extending out of the grooves are many delicate 

 tubes ending in suckers. These "tube feet" can be 

 lengthened and shortened. The animal attaches itself 

 to rocks by means of the suckers, and then pulls itself 

 along. It moves in any direction. At the end of each 

 ray there is a tentacle, which is probably an organ for 

 smelling, and on each tentacle there is a red spot, which is 

 sensitive to light, and is, therefore, a very simple eye. 



The exchange of gases (respiration) does not take place in all the 

 cells of the starfish, as in the simplest creatures, but in certain definite 

 spaces. These spaces thus form the beginnings of a breathing system. 

 The starfish is able to open the shell of the oyster, and to get at the soft 

 body within. It then turns a part of its stomach inside out over the 

 oyster, and digestion goes on outside of the body. The shell it simply 

 leaves behind. If a piece of a starfish is broken off, the cells that 

 remain produce new cells, and replace what has been lost. 



336. Worms. The student must not think of caterpil- 

 lars, grubs, and "measuring worms" as worms; they are 

 the larva stage in the development of insects (cf. 339). 



Dorsal Blood Vessel Muscles p _ 



Esophagus 



Pharynx 



[in 



FIG. 260. The Earthworm. Several segments are omitted. 



True worms do not change into anything else, but always 

 retain their "worm-like" form. Worms include earth- 

 worms (' ' angle worms ") , leeches, and many marine worms. 

 The most common of these is the earthworm (Fig. 260). 



