304 



GENERAL ZOOLOGY 



to some fixed object. Then begins the process of degener- 

 ation of all distinctly vertebrate 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. Some 

 zoologists have supposed that the 

 ancestors of the vertebrates were 

 worm-like animals. The reasons for 

 that supposition are based chiefly on 

 3 the structure of the acorn-tongue 

 worm, Balanoglos'sus (Fig. 151). This 

 animal is found in the sand of sea- 

 shores. It has a proboscis (Fig. 151, l) 

 with a collar-like band (Fig. 151, 2) at 

 the base, both Organs together some- 

 what 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 (Fig. 

 151, 3) open to the exterior. There is 

 a dorsal nerve-cord and a ventral nerve- 

 FIG. 151. Acorn-Tongue cord. A notochord-like structure has 

 Worm. Enlarged. (After ^ f d extending into the middle 

 A. Agassiz) & . 



ot the proboscis. Many zoologists 



1, proboscis ; 2, collar ; 3, gill- , , , . * , , . 



slits express doubt concerning this organ, 



and no investigator seems to be posi- 

 tive of its notochordal structure. If* the .main nerve-cords 

 were limited to one above the intestine, that would clearly 

 be a vertebrate characteristic ; but with one above and one 

 below, it is neither definitely vertebrate or invertebrate, and 

 hence possibly an intermediate form. The gill-slits have the 

 position and form of those in Amphioxus. 



