LOWER CHORD ATES 321 



cells. Stiffness is added to this rod by the cells being filled with liquid. 

 On the dorsal side of the notochord is the tubular nerve cord which pos- 

 sesses a small central canal and which is dilated at the anterior end to 

 form a rudimentary brain, known as a cerebral vesicle (Fig. 222). At the 

 anterior end of the nerve cord is an eyespot. The only sense other than 

 light perception known to be possessed by the amphioxus is that of touch. 



The circulatory system, which does not include a heart, is not more 

 complicated than that of the earthworm. As in vertebrates, however, 

 the course of the blood is the reverse of that in the earthworm, being 

 forward in a ventral vessel and backward in a dorsal one. Elimination 

 is carried on by a nephridial systern resembling that of some of the annelid 

 worms. 



Amphioxus lives buried in the sand with only the anterior end exposed. 

 The current of water rising from the atriopore serves to create a sort of 

 tube in which the body lies. In this position the oral hood and buccal 

 tentacles are opened wide to gather all the food particles possible. At 

 times the animals come out and swim around in the w^ater, particularly 

 at night and during the breeding season. When they bury themselves in 

 the sand, they enter head first and burrow very rapidly. 



Amphioxus is diecious. In the evenings of the breeding season, which 

 is in early summer, eggs and sperms are set free at the surface of the sea, 

 and there fertilization takes place. Cleavage is total and equal (Fig. 

 51), and a gastrula is produced by invagination. The mesoderm develops 

 from entodermal pouches. A free-swimming larva is produced which 

 gradually grows into an adult, at which time it begins to bury itself in 

 the sand. There is, therefore, no metamorphosis. 



342. Economic Value. — None of the lower chordates is of much 

 economic value except as laboratory material. However, in Science for 

 July 27, 1923, Light tells of the use of the amphioxus for food in China 

 and speaks of "a total of hundreds of tons of amphioxus taken during the 

 year" off the southern coast of that country. 



