STRUCTURE OF THE VERTEBRATES 15 



Cephalochordata. The cephalochordates are small, lance- 

 shaped animals living on sandy shoals of the ocean, near the 

 coasts of Europe, Asia and North America. The type form is 

 Amphioxus, its "double pointed" structure being an adaptation 

 to burrowing in the sand. The anterior end is braced by the 

 extension of the notochord into the rostrum which projects in 

 front of the mouth. The animals are active during the night, but 

 in the day burrow into the sand with only the anterior end pro- 

 jecting above the surface. In this position they wait for cur- 

 rents of water to wash food into their mouths. 



External Anatomy. The adult Amphioxus varies in length 

 from four to six centimeters, about two inches. The anterior 

 end can be determined by the pointed rostrum, and the ventral 

 mouth a few millimeters from the front. The latter is surrounded 

 by a circle of buccal cirri. In cross section the animal is roughly 

 triangular in shape, the base being the ventral side. 



In a preserved animal the oral hood and funnel can be seen 

 within the circle of cirri. The structures are formed by a fold 

 of tissue extending downward and backward from the rostrum, 

 so that a cross section through the hood would be circular. At 

 the bottom of the funnel is the mouth, which can be studied only 

 microscopically. The "fins" of the animal are simple in struc- 

 ture; a dorsal fin extends from the base of the rostrum to the 

 posterior end of the animal, where it becomes wider and swings 

 around the posterior tip to form the caudal fin, and the nar- 

 rower ventral fin. With the fins may be included the metapleural 

 folds, which will be discussed later. These folds are paired, 

 extending along the lateral ventral sides of the animal to a point 

 just anterior to the ventral fm, where the two folds meet. Exam- 

 ination will show that at the confluence of the folds there is an 

 opening, the atriopore. The anus can be located posterior to 

 the atriopore, on the left side of the ventral fin. 



The animal is covered with a thin skin, composed of an 

 ectodermal layer one cell thick, and a dermis, which is a loose, 

 gelatinous, mesodermal connective tissue. A non-cellular cuticle 

 is secreted by the ectodermal cells, acting as a transparent pro- 

 tective layer on the outside. Through the thin skin can be seen 

 the distinctly metameric, V-shaped muscle segments, separated 

 from each other by thin sheets of connective tissue, the 



