PROCHORDATA AND CYCLOSTOMES 



325 



unlike any vertebrate type in most of its characters. 

 The larva or first stage is minute and transparent, 

 and forms part of the plankton, or floating fauna 

 of the sea. It has no resemblance to the adult, 

 but does recall the larva of the Echinoderms, a fact 

 of considerable interest, because many naturalists 

 suppose that the whole prochordate series, leading in 

 one direction to the vertebrates, may have come from 

 an animal which belonged to the same group as the 

 ancestors of the starfish and sea urchins. 



5. The Amphioxus (more correctly called Bran- Amphioxus 

 chiostoma) derives its name from the fact that it is 

 sharp at both ends. The name is used for any one of 

 several similar species which burrow in the sand in 

 shallow bays. They are pallid creatures, shaped like a 

 small fish, the largest about 4 inches long. Of all the 

 Prochordates, they show most vertebrate characters. 

 They have a dorsal nerve cord, but no skull or brain ; 

 a well-developed notochord, but no vertebral column ; 

 a pharynx with gill slits, which do not, however, open 

 on the surface of the body, but lead to a chamber 





FIG. 118. Amphioxus (Branchiostoma lanceolatum). 



