278 



SYSTEMATIC ZOOLOGY. 



gradually the muscle of the oyster becomes fatigued, until 

 at last it can no longer hold the shell closed. Then the 

 starfish protrudes its stomach from the mouth, envelops the 

 flesh of the oyster with it, and thus digests the oyster out- 

 side of the body. The retractor muscles are to draw the 

 stomach back after a meal. This method of external eat- 

 ing explains the frequent degeneration or absence of 

 intestine and anus (p. 277). 



CLASS II. OPHIUROIDEA (BRITTLE- STARS). 



The brittle-stars, or serpent-stars as they are frequently 

 called, are much like the true starfishes, the chief distinc- 

 tions being that in the brittle-stars the arms and the disc 

 are sharply distinct from each other, and that the extremely 

 mobile arms are long, slender, and somewhat snake-like. 

 A little closer examination shows that the ambulacral 



groove has been carried into 

 the interior of the arms, and 

 that here one must search for 

 the ambulacral plates (fig. 

 93). There is no vent, and 

 the madreporite occurs on 



FIG. 92. Brittle-star (Ophiopkolis) 

 From Morse. 



FIG. 93. Qross-section of arm of 

 brittle-star, a, ambulacral plate; 

 ao, ambulacral opening. 



the lower side of the body, usually covered by one of the 

 plates surrounding the mouth. There are a few forms in 

 which the arms branch again and again, and since when 



