OHAPTEE X. 



CHOEDATA. 

 Tdnioates OB Sea-squirts and Amphioxus = Aorania. 



The Mollusca form the last true invertebrate group. Between 

 the invertebrates with their ventral nervous system and dorsal 

 haemal system and the true vertebrates with their dorsal nervous 

 system and ventral hsemal system and skeleton there are con- 

 necting-links — namely, the group of Aseidians, Tunicates, or Sea- 

 squirts ; a worm-like creature known as Balanoglossus ; and a 

 stage further, connecting these with the true definite vertebrate 

 animals, the little Lancelot, Amphioxus lanceolatum, a fish- 

 like creature found in the sands of the Mediterranean 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 here belong to the former section. 

 Amphioxus is undoubtedly a vertebrate animal; although it pos- 

 sesses no vertebrae, 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 only represented as mere pads 

 between the vertebrae in the Mammalia. 



