LOWER CHORD ATES 



319 



Free-swimming tunicates, either single or colonial, live in the surface 

 waters of the sea. They move about by forcing water out of the pos- 

 terior atrial opening or openings. Some colonial types form chains of 



F/n and fin-rays 

 Velum X =-/» 



^L^^43UJ'LVM^a^Ul^t^^^;x.;^;r^jJOl^^^V^^ . --_/ 



Pharnyngeal / Gonad 



Slifs / A 



Buccal 

 tentacles 



Metapleural folds 



Afriopore 



Anal _ 

 opening 



B 



Fig. 220. — ^An amphioxus. ^, seen from the side; B, from below. Some details of internal 

 anatomy indicated by dotted lines. From a specimen. X 2^^. 



individuals produced from a parent by budding, omitting the free- 

 swimming larval stage. 



The tunicates are the only chordates which exist in attached colonies. 

 There is an alternation of generations in the life cycle of some, a sexual 



■Dorsal fin 



Venfral 

 nerve 



Muscle 



Aorta 

 Coelom 



Gill 

 arch 



Gill 

 Slif 



Gonad 



Endosfyle 



Dorsal nerve 

 Nerve cord 

 Muscle 



Nofochord 



Nephridium 

 Pharynx. 



Liver 



Atrial 

 cavity 



Fig. 221. — Cross section through the gill region of amphioxus. {From Hertwig and Kings- 

 ley, "Manual of Zoology,'' after Lankester and Boveri.) 



generation, in which develops a larva that metamorphoses into a free- 

 swimming adult, being followed by an asexual generation, which repro- 

 duces only by budding. Thus these animals show retrogression in their 



