xiii BACKBONED ANIMALS 267 



youth, but mostly degenerate when adults ; and the 

 worm-like Balanoglossus and its allies may be plausibly 

 ranked as incipient Vertebrates. 



1. Enteropneusts.- -The term Enteropneusts (gut- 

 breathers) or Hemichorda is applied to Balanoglossus, 

 Ptychodera, and a number of other genera, of which it is 

 difficult to say whether they are worm-like Vertebrates 

 or Vertebrate-like worms. They occur in many parts 

 of the world, eating their way through the sandy mud 

 off the coasts. The body of Balanoglossus and its rela- 

 tives is ciliated and divided into distinct regions a large 

 ' proboscis " in front of the mouth, a firm collar behind 

 the mouth, a part with numerous gill-slits behind the 

 collar, and finally a soft coiled portion with the intestine 



FIG. 83. BALANOGLOSSUS, SHOWING PROBOSCIS, COLLAR, AND GILL-SLITS. 



and reproductive organs. The size varies from about 

 an inch to several feet, the colours are bright, the odour 

 is peculiar ; the sexes are separate. Enteropneusts are 

 especially remarkable in having a dorsal supporting rod 

 (like a notochord) in the " proboscis ' region, a dorsal 

 nerve-cord running along the back and especially 

 developed in the collar, and numerous gill-slits opening 

 dorsally from the anterior part of the food-canal. It 

 may be, however, that the short supporting rod of the 

 proboscis is only an analogue, not a homologue (see 

 chap. XI), of the notochord of the indubitable Verte- 

 brates, and the morphological importance of the dorsal 

 nerve-cord is considerably lessened when we discover that 

 there is a ventral one as well. Both are in fact but con- 

 densations of a network of nerve-cells and nerve-fibres 

 extending diffusely underneath the skin. Perhaps the 

 most satisfactory evidences of Vertebrate affinities are 

 to be found in the gill-slits and the way in which the 



