THE ANCEHTliY OF VEllTEBRATES 



293 



whose affinity with chordates is a little less certain. These 

 animals belong to the genus Balanoglossus (Fig. 278) ; they 



live in the mud of the seashore 

 and have doubtless lost many 

 organs as a result of their 

 burrowing habit. The elon- 



Or" 



Fig. 276. — Ciona, a simple 

 Tunicate, o, mouth ; at, open- 

 ing of atrium, or exhalent 

 opening ; si, stolon. After 

 Leuckart and Nitsche's dia- 

 gram. 



Fig. 277. — Two small colonies of 

 compound Tunicates (Botryllus). 

 The "zooids" are grouped about a 

 center, have a common cloacal 

 opening id), but separate mouths 

 (or). After Milne-Edwards. 



gated bodj^ is divisible into three parts : the proboscis at the an- 

 terior end ; the collar ; and the trunk. In the trunk an anterior 

 region containing gill-slits {hr, Fig. 279), and a posterior region 

 that is without gills can be chstinguished. The mouth lies 

 between the proboscis and collar and leads into a nearly 



