306 



GENERAL ZOOLOGY 



Proboscis 



openings, and a nervous system, but there is no indication 



of a notochord in the adult animal. 



The adult tunicate is simpler in structure than the larva. 



The larva has very much the form of a frog tadpole, and it 



swims about. Its locomotor or- 

 gan is a fin-like tail. In the 

 larva there are structures which 

 indicate vertebrate relationship. 

 Extending through the middle 

 of the tail is a notochord which 

 is evidently a supporting organ. 

 Dorsal to the notochord is the 

 nerve cord. Below the notochord 

 is the alimentary canal, and close 

 by are the beginnings of the 

 gills, with openings which, how- 

 ever, are not gill slits. When the 

 larva reaches a certain stage of 

 development, it fastens itself by 

 adhesive papillx to some fixed 

 object. Then begins the process 

 of degeneration of all distinctly 

 chordate structures; the tail is 

 absorbed, the notochord also 

 disappears, and the nerve cord 

 changes form. If it were not for 



what is known of the larva, the vertebrate relationship of 



tunicates would never be suspected. 



The Acorn-Tongue Worm. The acorn-tongue worm, Bala- 



noglos'sus (Fig. 158) is found in the sand of seashores. It 



has a proboscis with a collar-like band at the base, both 



organs together somewhat resembling an acorn in its cup. 

 The mouth, at the base of the proboscis, opens into a 



pharynx, from which many pairs of gill slits open to the 



Fig. 158. Acorn-tongue worm. 

 (Enlarged) 



Modified from A. Agassiz 



