CHAPTER XV 



The Echiuroids 



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{PJiyln nt Ec/i in i vldca) 



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.MONG the marine animals for whicii none of 

 the scientific pigeonholes seems adequate are some 

 sixty-odd whose general sausage shape as adults con- 

 ceals the fact that they originate, as do so many 

 mollusks and annelid worms, from a swimming troch- 

 ophore larva. The echiuroid proceeds, in fact, as 

 though to become an annelid, developing exactly 

 fifteen segments to the body. Then it loses almost all 

 evidence of these feature.-;, and takes up life either 

 buried in the mud or protected within the cavity of 

 some shell or coral rock. 



Echiuroids ordinarily hold their mouths at the 

 doorway of the burrow or hiding place. Beyond it, 

 into the sea, the animal extends a long, troughlike or 

 spoon-shaped prostomium and moves this about 

 while cilia on the concave surface gather detritus 

 from the bottom and pass it to the mouth. 



On both sides of the Atlantic Ocean and along the 

 Pacific coast of America from California northward, 

 Echiiinis pullasii reaches a length of 12 inches plus 

 the 4-inch prostomium. Echiunis echiurus of Eu- 

 rope is found in the same mud flats but makes a 

 U-shaped burrow and enlarges this as it grows. 



In Bonellia the prostomium is as much as 3 feet 

 long and forked toward the tip, but the living habits 

 and food-gathering procedure of the female are al- 

 most identical with those of Echiunis. Larval Z?o- 

 nellici appear able to develop into either sex, but all of 

 those that settle on the bottom become females. A 

 larva that chances to settle on the extended prosto- 

 mium of a female Bonellia becomes, instead, a male. 



The female echiuroid Bonellia viridis has a green-col- 

 ored oval body with a long, extensible proboscis, 

 forked at the tip. The minute male lives within her 

 body. ( France. Ralph Buchsbaum ) 



He remains microscopic in size and lives as a parasite 

 upon his mate, in her mouth or excretory organs or 

 reproductive tract. 



Urechis, an echiuroid of the intertidal zone, has a 

 different method of feeding. It uses its burrow as the 

 support for a finger-shaped tube of mucus cemented 

 to the rim of the burrow opening. Body movements of 

 the worm in the U-shaped burrow draw water through 

 the mucus filter, which strains out food particles. 

 After the mucus tube becomes loaded, pumping is 

 more difhcult. Urechis then swallows the tube with 

 its enclosed meal, moves to the burrow opening, and 

 attaches the rim of a new mucus filter. When the wa- 

 ter is charged with microscopic particles, Urechis 

 may swallow and replace its filter every two or three 

 minutes. But if the water is clear, a whole hour may 

 be needed to load a single mucus tube. 



[157 



