347 



known to occur is ])robal)ly due to the fact that after the 

 death of the animal the skeleton quickly falls apart into its 

 component plates, which become separately embedded in the 

 sediments. In the majority of the Asterozoa mero-plank- 

 tonic, bilaterally symmetric, ciliated larvjTp occur, which in 

 the Asteroidea are known as Bipinnaria and Brachiolaria, 

 and in theOphinroidea as Pluteus. These are often found in 

 great numbers in the pelagic fauna. 



Echinoidea. — The Echinoids, or Sea Urchins, are without 

 exception marine vagrant benthonic animals, living usually 

 in large numbers in moderate depths. A. few species 

 descend to depths between 2,000 and 8,000 fathoms, but 

 the majority prefer the shallow poi-tions of the littoral 

 districts. On the coast of Maine thousands of StrongyJocen- 

 trotiis drobachiensis are exposed at very low tides, lying 

 among stones and covered with fragments of shells and 

 with small pebbles. The Echinoidea delight in a sandy bot- 

 tom, from which they are brought up in vast numbers at 

 each haul of the dredge. Some species prefer fine mud, in 

 which they are often buried to some extent. When living on 

 rocks, they commonly bore holes for themselves, and even 

 the solid granite has been known to be thus attacked by the 

 animal. If corners and crannies are available, these are 

 often occupied by the animal in preference to a drilled hole. 



The larva of Echinoidea is known as a Pluteus, and is a 

 mero-planktonic, bilaterally s^-mmetrical, commonl}' more 

 or less ciliated organism, with a number of processes or 

 arms. It is often carried by marine currents to great dis- 

 tances, remaining in some cases afloat for several weeks be- 

 fore settling down. 



Holothuroiden. — The Holothurians are, like the Echinoidea, 

 marine benthonic organisms, but their habit of life is often 

 more sedentary than vagrant, the animals being buried in 

 the sand and mud, though never attached. Their bathy- 

 metri(; range is from the shore zone, where they may be dug 

 out of the sand at low tide, to the depths of the abyssal 

 district. Sandy or muddy bottom is usually preferred by 



