CHAPTEE X. 



CHORDATA. 



TUNICATES OE Sea-SQUIRTS AND AmPHIOXUS = AcRANIA. 



The MoUusca form the last true invertebrate group. Between 

 the invertebrates witli their ventral nervous system and dorsal 

 hcemal system and the true vertebrates with their dorsal nervous 

 system and ventral hsemal system and skeleton there are oon- 

 nectingdinks — namely, the group of Ascidiang, Tunicates, or Sea- 

 squirts ; a wormdike creature known as Balanoglossus ; and a 

 stage further, connecting these with the true definite vertebrate 

 animals, the little Lancelet, Amphiu.rus lanceolatum, a fish- 

 like creature found in the sands of the ilediterranean and in 

 Japan. These are called, collectively with the true Vertebrata, 

 by some modern zoologists Chordata. The Chordata are 

 divided into two sections — viz., (1) Acrania and (2) Oraniota. 

 The curious types we refer to hero belong to the former section. 

 Amphioxus is undoubtedly a vertebrate animal; although it pos- 

 sesses no vertebrsB, no true internal skeleton, yet, as we shall 

 see, it has a dorsal nervous system, and that structure peculiar 

 to chordate or vertebrate animals — namely, the notochord, which 

 becomes less and less recognisable the higher we proceed in 

 the vertebrate scale, until it is oidy represented as mere pads 

 between the vertebrae in the iSIammalia. 



