THE SMELT AND ITS ALLIES 251 



in earlier chapters are invertebrates. Between verte- 

 brates and invertebrates and connecting the fishes with 

 more worm-like ancestors, is the lancelet or Amphioxus. 1 

 This slender, rod-like animal is only 50 to 70 milli- 

 metres long. Its internal structure shows its relationship 

 with vertebrates ; it has the forerunner of a vertebral 

 column and a spiral nerve and it has also gill slits like a 

 fish, but it has no skeleton. 2 It lives in sandy seashores 

 in temperate and tropical zones. On our Eastern coast it 

 is found from Chesapeake Bay south. It stands embedded 

 in the sand, the tentacle-fringed mouth projecting above 

 the surface. It feeds on minute organisms of all sorts. 



Of the invertebrate groups, that of Tunicata lies nearest 

 to the stem from which the vertebrates arose. The adult 

 animals, however, are very different from vertebrates, for 

 they are attached (Fig. 236), and sometimes even form 

 colonies (Fig. 237). But the young animals are much 

 like tadpoles of frogs ; not merely superficially, but in 

 their structure. Going further back it seems clear that 

 vertebrates have developed out of the worms. A very 

 worm-like animal, which seems to foreshadow the verte- 

 brates in having gill slits and the forerunner of a back- 

 bone, is common in our sandy beaches. This is Balano- 

 glossus 3 the acorn-tongued worm, so called from the 

 shape of its proboscis (Fig. 238). 



i', both [endsj ; 6tfs, sharp pointed. 

 2 Fig. 235. 3 /SdAaws, acorn ; 7\(rcra, tongue. 



