Phylum Chordata 565 



oral hood are long cirri. The principal supporting structure of the body 

 is the notochord which extends almost throughout the entire length of 

 the animal. It consists of a gelatinous material surrounded by connective 

 tissue. 



The digestive tract is rather simple, beginning at the month which 

 is in the center of a membrane, the velum. Around the mouth are the 

 velar tentacles which keep large particles from entering. Just posterior 

 to the mouth is the large pharynx, which contains many diagonal gill slits 

 for the passage of water. These gill slits are visible externally. From 

 the pharynx a straight intestine leads to the anus. At the anterior part 

 of the intestine there is a large digestive gland. The pharynx which is 

 located in an external cavity, the atrial cavity, contains a mid-dorsal and 

 a midventral groove. The dorsal groove is the epihranchial groove, 

 while the ventral one is the endostyle. Both act to trap food in the man- 

 ner described for the endostyle of the tunicates. Water passing into the 

 pharynx goes through the gill slits which are supported by the gill bars 



MYOTOME 

 NERVE COBO 



EPCCRMIS 

 NOTOCHORD. 



EPI8BANCH1A1. 

 GROOVE 



GU-L SLIT 



TRANSVERSE 

 MUSCLE 



ATRIAL CAVITY 



METAPLEURAL FOLD 



COELOM 



.COELOM 



ENDOSTYLE 



Fig. 189. — Cross section of Branchiostoma in the region of the pharynx. 



