LOWER CHOR DATES 



313 



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 anaf fin-rays 

 Velum y c-/- 



HMljIII ^ lrtii-n-ir-i-i-ir|-_f \ i| m ; |,,^ , ____/' "' 



Pharnyngeal 

 slits 



Buccal 

 tentacles 



Gonad 



/ A 



Metapleural folds 



~~- . ;/:-,0rH^^T rIT'.I lTi-"£LllircLSijS SJ 



B 



Fig. 207. — An amphioxus. A, seen from the side; 5, from below. Some details of internal 

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



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 



Dorsaf fin 



Ventral 

 nerve 



Muscle 



Aorta 

 Coelom 



Gill 

 arch 



Gill 

 Slif 



Gonad 



Endostyle 



Dorsal nerve 

 Nerve cord 

 Muscle 



Notochord 



Nephridium 

 Pharynx 



Liver 



Atrial 

 cavity 



Fig. 208. — Cross section through the gill region of amphioxus. {From Herlwig 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 



