ORIENTATION Xvii 



of the digestive tract known as the pharynx. The pharyngeal area of the di- 

 gestive tract is composed of a series of paired skeletogenous arches known 

 as the visceral or branchial arches, between which are found the branchial 

 pouches and branchial furrows or grooves. 



B. Major Divisions of the Phylum 



The entire phylum Chordata may be divided into the lower chordates and 

 the higher chordates. 



Lower Chordata (Acraniata) 



Subphylum: Hemichordata 



These are small, soft-bodied animals living along the shores of the sea, and 

 in some instances to considerable depths into the sea. Dorsal and ventral 

 nerve cords are present in the class Enteropneusta or the "tongue worms." 

 The notochord is a short structure confined to the anterior end. Gill slits are 

 present. 



Subphylum: Urochordata (Tunicata) 



These forms inhabit the sea from the polar regions to the equator, and 

 from the shores outward to considerable depths. It is in the larval form that 

 this group lays most of its claim to a right to be placed among the Chordata, 

 for the young hatches as a larva which resembles the amphibian tadpole super- 

 ficially. In this tadpole a dorsal nerve cord is present, and in the tail region 

 a well-formed notochord as well. Gill slits also are found. Later in life the 

 larva settles down to a sessile existence and the tail with its notochord is lost. 



Examples: Styela partita; Molgula manhattensis; Ciona intestinalis. 



Subphylum: Cephalochordata (Lancets) 



To this group belong the familiar forms known as Amphioxus. Of all the 

 lower chordates, the lancets possess characteristics closely resembling the 

 higher chordate group. A dorsal tubular nerve cord is present, below which 

 is an elongated notochord, and an extensive pharyngeal area is developed. 

 The basic plan of the circulatory system resembles that of the vertebrate 

 group, although many pulsating "hearts" are to be found, one in each of 

 the numerous blood vessels coursing through the pharyngeal area. 



Examples: Branchiostoma virginiae; B. calif orniense; Asymmetron 



macricaudatum. 



Higher Chordata (Craniata) 



Subphylum: Vertebrata 



Group I: Agnathostomata 



To this group belong the cyclostomes or the vertebrates without jaws. The 

 cyclostomes include the lampreys (Hyperoartia) and the hagfishes (Hyper- 



