290 MINERALS AND GEOLOGY 



SUB-KINGDOM VIII. 



TUNICATA. 



The forms of this sub-Kingdom Tunicates or Ascidians were 

 classed until recently among the Molluscoidea, and are still placed 

 there by many authorities. They comprise a series of marine ani- 

 mals of small size, in which the body is enclosed in a thick, sack-like 

 integument. Some are simple, others compound organisms, at least 

 in the adult state. They have been raised to their present position 

 from certain very striking resemblances between the young ascidian 

 and the amphioxus or lancelet, first detected by Kowalewsky in 

 1866. As the amphioxus is now definitely regarded as a low form 

 of the fish class, the tunicates are thus looked upon as constituting a 

 connecting link between the invertebrata and higher types. Apart 

 from this, they have no palseontological interest, fossil forms being 

 entirely unknown. 



SUB-KINGDOM IX. 



VERTEBRATA. 



This sub-Kingdom includes all animals of bilateral symmetry, pos- 

 sessing a vertebrated backbone, or a permanent chorda dorsalis. or 

 both. Limbs may be present or absent. Vertebrates thus comprise, 

 typically, all animals with internal cartilaginous or bony skeleton. 

 The only exception is the gelatinous Amphioxus or Lancelet ; but a 

 chorda dorsalis is present in this peculiar type, as in all embryonic 

 and some adult vertebrates.* The sub-Kingdom is separated into the 

 following classes: 1, Fishes; 2, Amphibians; 3, Reptiles; 4, 

 Birds ; 5, Mammals. The annexed diagram shews the life-range and 

 relative importance at different epochs, so far as revealed by fossil 

 evidence, of these various Classes of Yertebrata. 



* The chorda dorsalis or notochord is the forerunner or embryonic representative of the 

 vertebral column. In some fishes it is retained permanently, but in the great majority of ver- 

 tebrates it is merely a transitory organ. 



