322 MANUAL OF ZOOLOGY sect. 



of an Ascidian, being of similar shape, with a rounded body and a long 

 tail-like appendage attached to the ventral side, and with a distinct 

 notochord. This, however, is an adult animal, known as Appendicuia- 

 ria. It never becomes fixed and retains permanently its chordate 

 characteristics. 



OT" 



Fig. 202. — BotryllUS violaceus. or, oral apertures; cl, opening of common cloacal 

 chamber. (After Milne-Edwards.) 



A number of other Urochorda are permanently free-swimming, but 

 these are all almost, if not quite, as thoroughly metamorphosed as the 

 Ascidians, so that their true affinities only become clear when their 

 life-histories are followed. 



3. THE VERTEBRATA 



The Sub-phylum Vertebrata comprises the lancelets, the 

 lampreys and their allies, the fishes, the amphibians, the 

 reptiles, the birds, and the mammals. The lancelets occupy 

 an extremely isolated position with regard to the other mem- 



