THE LOWEST VERTEBRATES 155 



not for our knowledge of its early stages its real relation- 

 ship to the vertebrates would not be apparent. As in 

 the barnacles, and especially such degenerate forms as 

 Sacculina, the clue to the real affinities of these animals 

 was first revealed through a study of development. 



There is another group represented by Balanoglossus 

 and a few related genera which also show relationships 

 to the vertebrates. Balanoglossus is a worm-like animal 

 which burrows in the mud of the sea bottom by means of 

 a muscular proboscis. In the anterior part of the body 

 there are a number of gill-slits which lead from the pharynx 

 to the outside and serve for the exit of water taken in 

 through the mouth. There is a dorsal nerve cord and a 

 structure dorsal to the pharynx which is regarded as repre- 

 senting the notochord. Balanoglossus also resembles 

 the vertebrates in many features of its early development, 

 but its relationship is much less close than that of the 

 tunicates. 



In the lancelet, or Amphioxus, the vertebrate characters 

 are much more conspicuous. This animal, which Pro- 

 fessor Haeckel has called the most interesting vertebrate 

 next to man, has a narrow, laterally flattened body taper- 

 ing toward both ends. It lives in the sea partly buried in 

 the sand in which it can burrow, when disturbed, with 

 remarkable quickness. Like the tunicates its food con- 

 sists of small bodies swept into the alimentary canal 

 by means of cilia. The water that is taken into the mouth 

 passes out through numerous fine gill-slits in the wall of 

 the pharynx, while the solid particles are retained and 

 swept by ciliary action into the intestine. The backbone 

 is represented by a firm rod-shaped notochord extending 

 above the alimentary canal. Dorsal to the notochord lies 

 the nerve cord, the anterior end of which is slightly en- 

 larged to form a sort of brain. This nerve cord corre- 



