ECHINODERMATA 161 



be cut into several pieces each one will grow into a com- 

 plete worm. 



The largest of the flatworms are marine and belong to 

 the group Nemertini. Some of these reach a length of 

 nearly one hundred feet. The common form found along 

 the Atlantic coast is Cerebratulus, which has a flat, cream- 

 colored body two or three feet long. It inhabits the mud 

 and sand at the water's edge where it may be dug out during 

 low tide. 



The Rotifers are interesting microscopic worms present 

 in stagnant water. They belong to the subkingdom of 

 Trochelminthes. Upon the head are two circles of cilia 

 so vibrating to draw in food as to resemble the rotation 

 of a wheel. In early summer they are parthenogenetic, 

 i.e., young are produced from eggs which have not been 

 made fertile by the entrance into them of the sperms (re- 

 productive cells of the male). 



12. ECHINODERMATA 



This phylum derives its name from the rough or spiny 

 skin common to many of the species, such as the starfish 

 and the sea urchins. All of the members of this group 

 dwell in salt water. 



The Starfish (Asterias vulgaris) . Of the several species 

 of starfish living at depths of from one to three hundred 

 feet, the larger forms should be chosen for study. They 

 are numerous in the vicinity of oyster beds as they prey 

 on these shellfish, causing great loss to the oyster industry. 



The normal starfish has five arms of equal length, but 



