208 ZOOLOGY. 



gradual increase in the importance of the secondary 

 function. 



Equally remarkable is the fact that the thymus begins 

 its development as a series of projections on the dorsal side 

 of the gill-slits, closely resembling the first appearance of 

 tongue-bars in Amphioxus. Here again we have to do 

 doubtless, with a case of an organ of changed function. 



16. Classification. It is evident that between Amphi- 

 oxus and the dogfish there is a great gap at least as great 

 as would be the gap between dogfish and rabbit, were no 

 intermediate types known. Some few lowly fish, as the 

 lamprey, come a little way nearer Amphioxus than does 

 Scy Ilium, but it is only a little way. On the other side 

 the gap between Amphioxus and the nearest Invertebrates 

 is almost as great. The peculiar group of the Tunicates 

 does, it is true, show considerable affinities to Amphioxus, 

 but after that there comes a very great gap indeed. Hence 

 modern zoologists unite the Tunicates, Amphioxus, and typical 

 Vertebrates into one great phylum or division of the animal 

 kingdom, to which the name YERTEBRATA is given. Other 

 zoologists prefer to term this group CHORDATA, and restrict 

 the term " Vertebrata " to those forms higher than Amphi- 

 oxus. Whichever name we adopt, the following are the 

 essential characters that mark off this great phylum from 

 all other phyla of the animal kingdom : 



(1) At some period of existence there is a Notochord. 



(2) The central nervous system is dorsal in position, is a 

 continuous mass of nerve-tissue, and is tubular (i.e. has 

 a central canal). 



(3) The anterior portion of the alimentary canal gives 

 rise to the respiratory organs (aquatic or aerial), and at 

 some period of existence the pharynx is perforated by 

 gill-slits. 



(4) There is a general metamerism of the body-muscles 

 (myomeres). 



If we exclude the Tunicata we may add a fifth point. 



(5) The circulation shows primitively 3, hepatic-portal 

 syste n, and a median ventral trunk sending off branchial 



between the gill-slits, which unite in a dorsal aorta. 



