THE PHYLA HEMICHORDATA AND CHORDATA 



Muscle segments 

 Nerve cord / \ Intestine 



Notochord 



Anus 



Oral cirri 



Pharynx Liver 



General structure 



Atriopore 



Excretory organs 



Dorsal blood vessels 



Ventral blood vessel 



Ventral blood vessel Gonads Liver 



Circulatory system 



Nerve cord 



Nerves Nerve cord 



Notochord 



Dorsal fin 



Dorsal 

 vessel 



Atrium 



Atrium 



Lateral ridge Transverse sections 



Region of pharynx Region of intestine 



Fig. 18.6. Subphylum Cephalochordata : general structural features of Branchtostoma, the 

 amphioxus or lancelet. 



gonads. The mouth is located anteriorly within a hood-like structure, the 

 edge of which forms a fringe of delicate processes called cirri. The anus is 

 located on the left side, near the base of the tail fin. Another opening, the 

 atriopore, lies just anterior to the ventral fin; it serves as the excurrent 

 opening for water that has passed through the mouth and gill slits. The 

 metameric arrangement of the musculature is conspicuous and may be seen 

 through the thin epidermal covering of the body. 



The digestive tract consists of a pharynx, or branchial sac, perforated by 

 a series of paired gill slits, and an intestine leading posteriorly to the anus. 



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