PROCHORDATA AND CYCLOSTOMES 323 



contract, emitting a stream of water. The mouth 

 leads into a large sac, the pharynx, the walls of which 



nerve ganglion 



mouth 



nerve cor 



notochord 



Drawing by W. P. Hay 



FIG. 116. Diagram of the anatomy of Appendkularia, one of the Larvacea: 

 a, lateral view ; b, cross section of tail. 



have a more or less latticelike structure, with many 

 small openings. This is the gill apparatus, and the water 

 passing through it gives up its dissolved oxygen to the 

 blood. At the end of the pharynx is the opening of the 

 alimentary canal. The pelagic or free-swimming Tuni- 

 cata are very different - - more or less cylindrical, and 

 transparent. Some are quite large, others minute. 

 These animals were formerly associated with the Mol- 

 lusca and regarded as a sort of shell-less clams, but the 

 investigation of their immature stages showed the entire 

 error of this view. The larva or young stage is a more The larval 

 or less tadpolelike creature, with a long tail containing a ^^ 

 notochord. In development, all this gradually disap- 

 pears by absorption, except in a group of minute free- 

 swimming forms constituting the class Larvacea, which 

 retain the elongate form and notochord through life. In 

 addition to the characters mentioned, the tunicates have 

 a dorsal nervous system, so that in several important 

 respects they are to be associated with the vertebrates 

 rather than with the typical invertebrate animals. 



It is commonly said that the tunicates exhibit de- 

 generation. This is not quite exact, but it is true that 

 after seeming in the early stages to promise develop- 



