A NIMALS WITH SPINY SKINS— THE ECHINODERMS 293 



must relax and the shell opens. Then the starfish does a very strange 

 thing ; it everts its stomach through its mouth and wraps it around the 

 soft body parts of the oyster and pours out powerful digestive juices that 

 digest the oyster right in its shell. The digested oyster is absorbed 

 partly through the stomach wall and partly by the pyloric caeca and 

 the liquefied food passes into the coelomic fluid by which it is distributed. 

 Retractor muscles extending from each arm to the stomach pull it back 

 into place and the animal is then ready to seek another meal. A single 

 starfish may destroy ten or twelve oysters a day in this way and, need- 

 less to say, starfish are great enemies of the oyster fishermen. Vari- 

 ous steps are taken to combat their presence in oyster beds ; one common 

 method is to drag the ends of frayed rope or similar structure over the 

 bottom. The starfish will grasp these objects with their pedicellariae 

 and are thus lifted to the surface. At one time they were chopped in 

 two and thrown back in the water, but, because of regeneration, this 

 actually doubled the population. Now they are collected and dried so 

 that death is certain. 



There are many tiny mollusks which may also serve as food ; these 

 are taken into the stomach through the mouth and, after digestion, the 

 shells are spit out through the mouth. 



The anus of the starfish is used very little, since for the most part 

 only digested or easily digested particles are taken into the digestive 

 system. When small mollusks are ingested, their shells are more easily 

 discharged through the mouth on the underside. The ancestors of the 

 starfish, in all probability, had a functional anus; and, although still 

 present, it is so greatly reduced in size that it is regarded as a vestige. 

 The secretion of a plentiful supply of digestive fluids reduces the time 

 that a starfish has to sit on an oyster with its stomach protruding. 

 This is accomplished by five large pairs of pyloric caeca that fill the 

 coelom of each of the five arms. 



The nervous system includes a nerve ring around the mouth and a 

 nerve running out into each arm with smaller branches connecting with 

 the different parts of the body. Sense organs are poorly developed, 

 but at the end of each arm there is a pigmented eyespot which is sensi- 

 tive to light and both the skin gills and pedicellariae are sensitive to 

 tactile stimuli. 



Reproduction is quite simple ; sexes are separate and a pair of ovaries 

 or testes lies in each arm between the pyloric caeca and the ampullae. 

 Their size varies considerably with the degree of maturity of the animal 

 and the season of the year when they are gathered. These gonads open 

 to the outside by small pores at the points on the central disc between 

 the arms. Sperms and eggs are shed into the water during the reproduc- 



