//. TYPES IN DETAIL IVITH THEIR SUBORDINATE GROUPS- 37 



gill-slits. The typical members of 

 this class are solitary and sessile, 

 with the two orifices approximated. 

 Some are colonial with certain 

 parts in common, and in a few of 

 these cases the colony is free-swim- 

 ming and pelagic. 



Order i. Ascidi^ Sessile forms, either solitary (mona- 



scidicB), or colonial {synascidicB), 

 and arranged in generally stellate 

 groups, known as canobia. 



X Boltenia, Botryllus. 



Order 2. Pyrosomin^ Free-swimming, transparent colo- 

 nies of a cylindrical or cone- 

 shaped form. Incurrent open- 

 ings upon the exterior, cloacal 

 openings in the interior. 



X Pyrosoma. 



Sub-class III. THALIACEA Free-swimming, pelagic forms, 



transparent and surrounded by a 

 gelatinous tunic. Pharynx with 

 either two rows of gill-slits or with 

 a single pair of large gill-slits. 

 Oral and cloacal openings at oppo- 

 site poles. The members of this 

 group often show a remarkable 

 alternation of solitary and colonial 

 generations, or even a still more 

 complicated development. 



Order i. Dolioli^ Two rows of gill-slits. Muscle 



bands in the form of closed rings. 

 Mantle thin. Generative cycle 

 complex, including one sexual and 

 two asexual generations. 



X Doliolum. 



Order 2. SalpIjE A single pair of gill-slits. Muscle 



bands not in the form of complete 



