306 CLIMAX OF THE ANIMAL KINGDOM— PHYLUM CHORDATA 



longitudinal folds of skin (metapleural folds) extending most of the 

 length of the body, and it is thought that these may be forerunners of 

 paired appendages. It also has median dorsal and caudal fins. The 

 notochord runs from one end of the body to the other and gives a cer- 

 tain degree of rigidity to the body. The muscles are V-shaped seg- 

 ments known as myotomes which extend the entire length of the body. 

 The hollow spinal cord lies just above the notochord, but the brain is 

 very poorly developed and actually contains less nerve cells than an 

 equivalent portion of the spinal cord. There are no definite eyes, but 

 a row of pigment spots along the spinal cord are sensitive to light. 



Amphioxus often lies half buried in the sand with only the oral 

 hood protruding. Buccal cirri line the oral hood and prevent sand 

 from entering the body. Water is drawn through the oral hood and 

 passes through the mouth into the pharynx due to the action of cilia. 

 The water then passes out of the pharynx through numerous gill slits 

 into the atrium, a cavity which surrounds the pharynx, and finally flows 

 back through the atriopore, which is an opening that might be com- 

 pared to the excurrent siphon of the clam. Particles of food in the 

 water are caught in a sticky mucus in a groove, known as the endostyle 

 which lies on the ventral border of the pharynx. From here the food is 

 carried back into the intestine where it is digested with the aid of the 

 liver diverticulum, a ventral outgrowth from the intestine. 



Sexes are separate, and gametes are produced by 26 pairs of gonads 

 which are found in the atrium. Amphioxus is most like the vertebrates 

 of all the chordates, but since it does not have even the rudiments of a 

 vertebral column, it must rely upon the notochord for support of its 

 muscles. 



Subphylum IV. Vertebrata. This group includes the remainder of 

 the chordates. They take their name from the vertebral column which 

 consists of bony articulating vertebrae surrounding the spinal cord, 

 which is found in all but the most primitive vertebrates. 



Classification and Derivation of Scientific Words 



Phylum Chordata (G. chorde, cord; refers to the notochord that is charac- 

 teristic of this group). 



Subphylum I. Hemichorda (Gr. hemi, half; chorde, cord; have a noto- 

 chord only in the anterior end of the body). Example, acorn 

 worms. 



Subphylum II. Urochorda (Gr. oura, tail; chorde, cord; larvae have 

 a notochord in the tail). Example, sea squirts. 



