200 ZOOLOGY 



Not less important are the points in which chordates resem- 

 ble other animals. First of all they are bilaterally symmetrical 

 animals, at least in their young stages ; secondly, they are seg- 

 mented animals, and they have ventral organs repeated as in 

 other externally ringed animals. Consequentlj^ they show 

 their affinities to the great groups to which annelids and ar- 

 thropods also lielong. A diagram showing the relationship 

 of chordates to the invertelDrate groups already studied would 

 he something like this : 



Chordata 



Arthropoda / 



Annelida 



Eehinodermata MoUusca 



Scolecida ' 



' Coelentorata 



Porifera 



Protozoa 



Appl}'ing now the four criteria of chordates enumerated 

 above, we fiutl that they discover strange associates for the 

 vertebrates. Of these the most remarkable is a group long 

 known as Tunicates, animals which in some cases are attached 

 and in others float on the surface of the sea. Tunicates show 

 a great variety of external forms (Fig. 274), but they all re^•eaI 

 their chordatc nature in their youthful forms. Thtis in the 

 tadpole stage (Fig. 275) we see the chorda (noto) in the axis 

 of the bod}', the nerve-cord (tned) above it ; and one of the 

 throat slits (sfi(j) already formed. When the animal attaches 

 itself, it loses its tail and chorda, but the number of gill-shts 



^ Includes flatworms, roundworms, rotifers, bryozoans, aud brachiopods. 



