CLIMAX OF THE ANIMAL KINGDOM— PHYLUM CHORDATA 



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in its cup. The rest of the body, the trunk, is long, slender, and somewhat 

 flattened. The trunk bears numerous gill slits on its anterior end up near 

 the collar. The collar, which really corresponds to the head, contains the 

 mouth and a primitive brain. Behind the brain is a nerve cord, which is 

 somewhat tubular in structure, but there is some question as to whether 

 it should be considered a true tubular nerve cord. A short strand of 

 cells extends from the collar into the proboscis. This group of cells 

 forms what is usually called a notochord, but, because it does not extend 

 posteriorly under the nerve cord, this is also open to question. The 

 larva of the acorn worm is of significance because it is so similar to 

 some larvae of the echinoderms. Before the metamorphosis of this 

 (tornaria) larva was observed, it was thought to be an echinoderm. 



Dorsal fin 



Body wall 



Notochord 



Velar 

 tentacles 

 Buccal cirri 



Branchial bars 

 of pharynx 



Atrium 

 Gonads Atriopore 

 Liver diverticulum 



Caudal fin 



Foundations of Biology, Woodruff, The Macmillan Company 

 Fig. 21.5. Amphioxus, with part of the body wall removed to show the internal organs. 



This strengthens the theory of a connecting link in the ancestry of the 

 chordates and the echinoderms. 



Subphylum II. Urochorda. This group includes the sea squirts or 

 tunicates which are not much more than flexible little bags attached at 

 one end and sucking water in and squirting it out at the other end. They 

 do not seem to have much in common with the other members of the 

 phylum, but their larvae are free-swimming forms and have a notochord 

 in the proper relation to the dorsal nerve cord and several other chordate 

 characteristics. 



Subphylum III. Cephalochorda. The most important genus of this 

 group is Amphioxus, a little marine animal that is found near the shore 

 line of sandy beaches in certain tropical regions of the world. It is an 

 excellent specimen for advanced study in working out the beginnings of 

 vertebrate characteristics and is an animal with which all students 

 of comparative vertebrate anatomy soon become thoroughly familiar. 

 Amphioxus is shaped somewhat like a small, slender flsh. It has two 



