Chap. 33 



VERTEBRATES LOWER CHORDATES AND FISHES 



665 



PROBOSCIS COLLAR 



MOUTH CILLS 



Proboscis 



_ Dorsal 



Collar nerve cord 



(Notochord) 



Gill slits 



Fig. 33.3. Representative lower chordates. A notochord is present at some 

 time in every chordate; in man only in the embryo. Upper, acorn "worm," 

 Balanoglossus (adult worm), a burrower in sand, between the tides and deeper. 

 Middle, a sagittal section of the anterior end of the adult. If the notochord were 

 prolonged backward it would lie between the nerve cord and the alimentary canal 

 as it does in all chordates. Lower. Amphioxus — Branchiostoma lanceolatus. About 

 2 inches long, a burrower along the coasts of warmer seas. ( Upper, after Bateson. 

 Courtesy, Rand: The Chordates. Philadelphia, The Blakiston Co., 1950. Middle, 

 courtesy, Romer: The Vertebrate Body. ed. 2. Philadelphia, W. B. Saunders Co., 

 1955. Lower, after Hesse-Doflein: Tierbau iind Tierleben. Leipzig, B. G. Teubner, 

 1910.) 



