water itself. They emerge, wriggle a few inches, and 

 dive in again so quickly that one naturalist realized 

 he could not tell whether they swam mouth forward 

 or tail first. To learn the answer, he caught some 

 lancelets and carefully dipped the tail of each into a 

 harmless dye. Then he released them in an aquar- 

 ium with a sandy bottom. Some dove into the sand 

 mouth first. Others went tail first. But when they 

 swam around of their own volition, the mouth was 

 always in advance. 



Undisturbed lancelets rest in the sand with just the 

 mouth exposed. Water drawn into the pharynx passes 

 through oblique S-shaped slits into the atrium and 

 emerges into the sand about two-thirds of the way 



along the ventral surface. The anus opens farther 

 back, at the base of the narrowly fin-bordered tail. 

 The reproductive organs, either testes or ovaries, 

 form a series of block-shaped bags that bulge into the 

 atrium and release their products into the water 

 being discharged through the atrial opening. Fertili- 

 zation occurs outside the body, and the development 

 of the embryo follows a pattern closely comparable 

 to that in many vertebrates. The early stages, how- 

 ever, suggest the steps in growth of an echinoderm 

 embryo. For this reason, lancelets have been of spe- 

 cial interest to scientists, and are still known by an 

 outmoded generic name as "amphioxi." "Amphi- 

 oxus" merely indicates "pointed at both ends." 



[289 



