'XII 



PHYLUM CHORDATA 



321 



or- 



of these compound forms (Fig. 189), distinguished as the Composite 

 Ascidians, the tests of the zooids are united together to form a mass 

 of gelatinous consistency in 7 



which the zooids of the 

 colony lie embedded. 



A minute animal which 

 swims about in the surface 

 waters of the sea has in 

 most respects an extremely 

 close resemblance to the 

 tailed larva 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 Appendiciilaria. 

 It never becomes fixed and 

 retains permanently its chor- 

 date characteristics. 



A number of other Uro- 

 chorda 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. 



FIG, 189. Botryllus violaceus. or. oral aper- 

 tures ; cl. opening of common cloacal cham- 

 ber into which atrial apertures of zooids 

 ,open. (After Milne-Edwards.) 



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 members of the sub-phylum, and are best regarded 

 as constituting by themselves a division, which for reasons 



Man. Zool. Y 



