PHYLUM CHORDATA 



319 



gut. Although it secretes digestive enzymes, 

 it is not known to function as the vertebrate 

 liver. The intestine leads directly to the 

 anus. 



Respiratory system 



The pharynx (Fig. 202) is attached 

 dorsally and hangs down into a cavity called 

 the atrium. The atrium is not the coelom. 



Epidermis 

 Fin ray 



Myotome 

 Dorsal root nerve 

 Nerve cord 

 Central canal 

 Ventral root nerve 

 Notochord 

 Sclerocoel (coelom) 



Dorsal aorta 

 Hyperbranchial groove 

 Coelom 



Protonephridium 



Gill bar 



Gill slit 



Pharynx 



"Liver 



Egg in ovary 



Coelom 



Transverse muscle 



Atrium 



Hypobranchial groove 

 Subendostylar coelom 

 Ventral aorta 

 Metapleural fold 

 Metapleural cavity 



Figure 202. A cross section of the amphioxus in the pharyngeal region, showing various in- 

 ternal structures, including some of the coelomic cavities. 



but it is lined with an ectodermal epithe- 

 lium and is really external to the body, as 

 has been proved by the study of its de- 

 velopment. Water, which is carried into 

 the pharynx by way of the mouth, passes 

 through the gill slits into the atrium and 



out of the atriopore. The gill slits are sepa- 

 rated by gill bars; these are ciliated and 

 supported by rods of connective tissue. 

 Respiration takes place as the water, driven 

 by the cilia, flows through the gill slits; the 

 phar)'ngeal mechanism and gill slits are 



