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TEXTBOOK OF ZOOLOGY 



a small fish, but it does not have a distinct head. The mouth opens 

 on the ventral surface of the anterior portion of the body. It is 

 beneath a rostrumlike projection and is nestled well up in an oral 

 hood which is shaped like an inverted funnel. This hood is fringed 

 with sensory fingerlike oral tentacles. There is a median fin along 

 the dorsal side, continuing around the tail as the caudal fin and 

 anteriorly about one-third of the length of the body as the ventral 

 median fin. There are no clearly defined lateral fins, but a pair of 

 skin structures, the metapleural folds, extending along the anterior 



Dorsal fin 

 F/n ray 



Epidermis 



.^ Spinal nervz 



\ Nerve cord 



^j Nobochord 



S Myoto;ne muscle 



\ Myocomma 



>] Dorsal Aorta 



^K— =■■ A^-Epibranchial qroove 

 .Nephridium 

 ^ Atriaicavity 



_ _ Liver 



Neurocoek '/^-^ 



NotochordoL _ '/ 

 sheath 



Coelom 



Atrial cavity- 

 Pharynx 



Gil/rod 



Gill bars 



Qonad 



Ventral aorta. 



Hypobranchial qroove 



Endostyle 



Coelom 



Metapleural fold 



S. 



Fig. 215. — Cross section of Amphioxus thirough the level of the posterior portion of 



the pharynx. 



two-thirds of the ventral surface of the body are thought to be their 

 forerunners. The ventral and dorsal fins are supported by small 

 vertical rodlike fin rays. On the ventral side, just posterior to the 

 metapleural folds, is an opening, the atriopore, and beside the ventral 

 margin of the caudal fin is the anus. The segmental divisions of the 

 muscles are apparent on the body wall. There are from fifty-eight 

 to sixty-four of them on each side in B. lanceolatus but sixty-nine 

 in B. calif orniense and they are known as myotomes. The myotomes 

 on the two sides are not paired, but alternate with each other. 

 Adjacent ones are separated by a myocomma or myoseptum. 



