ECHINODERMA. 293 



developed ambulacra), and with these the animals obtain 

 their food. Inside, the pharynx is surrounded by calcareous 

 plates, the whole resembling slightly the lantern of the sea- 

 urchin, but no teeth are ever developed. In most species 

 the madreporite is inside the body, and in many the bran- 

 chial trees (p. 285) become developed into large tree-like 

 structures. Along our shores two groups or orders occur; 

 PEDATA, in which there are ambulacra and branchial 

 trees; and APODA, in which both these structures are 

 lacking, and the body is decidedly worm-like. 



CLASS IL ECHINOIDEA (SEA-URCHINS) (see p. 288). 

 CLASS III. ASTEROIDEA (STARFISHES) (see p. 285). 

 CLASS IV. 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. There 

 is no vent, and the madre- 

 porite occurs on the lower 



FIG. 131. Brittle-star (Ophiopholis). FIG. 132. Cross-section of arm of 

 From Morse. brittle-star, a, ambulacral plate ; 



ao, ambulacral opening. 



side of the body, usually covered by one of the plates sui> 



