154 



ESSENTIALS OF ZOOLOGY 



on the ventral surface of the anterior portion of the body. There are 

 no clearly defined lateral fins, but a pair of skin structures, the 

 metapleural folds, extending along the anterior two-thirds of the ven- 

 tral 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, and they are known as myotomes. The myotomes 

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

 cent ones are separated by a myocomma or myoseptum. 



Dorsal fin 



Fin ray 



Epidermis 



5pinal nerve 



Nerve cord 



- -Nobocliord 



Myofco.'ne musde 



tiyocomma 



Dorsal Aortxj 



-Epibranchial qroove 

 — Nephridium 

 — AtriaKavity 

 ^ -.QUI slit 

 Liver 



Fig. 50. 



Neurocoele .^^^^s?:. 



Notocbordol- 

 sheath 



Coelom j 



Atrial cavity- 

 Pharynx 



Gill rod 



Gill bars 



Qonad 



Ventral aorta . 



':l;^.Jiypohrorich\al qroove 



0i E ndostyle 



U4 Coelom 



_ ^ _ Metapleural fold 



-Cross section of Amphioxus through the level of the posterior portion of 



the pharynx. 



Internal Structure and Metabolic Activities. — The notochord ex- 

 tends the length of the body as a slender rod of vacuolated cells 

 which are filled with fluid to give it turgor or stiffness. Immediately 

 dorsal to this rod is the nerve cord, which also runs the length of the 

 body. It has a small central canal or neurocoele extending length- 

 wise through it and is dilated at the anterior end to form the cerebral 

 vesicle or rudimentary brain. A mass of dark pigment is located at 

 the anterior end which is known as the eyespot. There are smaller 



