78 AN INTRODUCTION TO ZOOLOGY. 



gave rise at the time to. a good deal of harmless fun, 

 when a clever parody on a well-known song traced 

 the drinking propensities of the human subject to 

 its descent from " the Leather Bottel." But we now 

 recognise clearly that the ascidian and the vertebrate 

 can only be collateral offshoots from some parent form, 

 now lostj or as yet undiscovered in the depths of the 

 sea. 



A word should be said here as to the present state 

 of knowledge regarding links between the vertebrates 

 and invertebrates. If we hear less about the Ascidian 

 than formerly, it is not that it is now deemed less 

 really important. It is almost impossible for us now 

 to realize the change made by the discovery of a link 

 which connected the vertebrates with lower forms. 



The notochord is also found in a link which has 

 attracted more recent attention, a worm-like marine 

 animal called Balanuglossus. It has a larval stage 

 which resembles that of the Sea-urchin ; and when 

 grown up, it has a row of gills to breathe through, 

 which strongly remind one of the breathing apparatus 

 of.ascidians (Fig. 86, page 238). 



But it is not only by the discovery of new forms 

 that we get light on the relationships of the back- 

 boned animals, but quite as much by the careful 

 examination and reconsideration of forms already 

 known. If there is one thing we consider more 

 distinctly characteristic of the back-boned animals 

 than anything else, it is their possession of paired 

 eyes and ears j but it was announced a few years ago 



