866 TEXTBOOK OF ZOOLOGY 



legs are jointed, although similar in appearance to paropodia of 

 Anjielida. There are two jointed antennae on the head and some 

 jawlike plates in the mouth. Situated as it is, midway between 

 annelids and arthropods, this form seems to show an immediate 

 transition from the one group to the other. Upon this basis it is 

 usually held that the arthropods have a wormlike ancestry. 



Interestingly enough, there exists another idea of arthropod an- 

 cestry from the fact that all of the lower forms of Crustacea pass 

 through a characteristic stage knoAvn as the Nauplius. This larva 

 does not correspond very closely to any strictly annelid stage, but 

 with its short body and three pairs of appendages it resembles a 

 modified trochophore larva. The nauplius larva has some features 

 in common with the rotifers which authorities feel may have arthropod 

 tendencies. 



Echinoderms and Their Larval Relations 



Although the adult echinoderms possess radial symmetry, that 

 seems not to have been in the immediate phylogenetical background. 

 The larvae of all echinoderms possess bilateral symmetry. These 

 larvae all have definitely arranged bands of cilia over the body. 

 Embryologists have pointed out the fact that these larvae are not 

 directly related to the trochophore. They show more resemblance 

 to the Tornaria larva of Hemichordates than to any of the non- 

 chordate forms. 



Ancestry of the Vertebrates 



The establishment of relationships between the chordate and non- 

 chordate animals has been one of the perplexing problems in the 

 study of phylogeny. Different students of this problem have in- 

 vestigated the possible relationships of such nonehordate groups 

 as the flatworms, nemertine worms, annelids, arachnids, insects, and 

 echinoderms. Their investigations have resulted in the formulation 

 of a number of theories putting forth the various ones of the above- 

 mentioned groups as the progenitors of, or claiming common an- 

 cestry with, chordates. 



Some authors insist that the chordates have arisen from some 

 segmented form ; others conclude from their evidence that this is 

 not necessarily true or essential. All of the theories establish their 

 relationships to the vertebrates through the protochordates, which 

 are represented by Amphioxus, the tunicates, and hemichordates. 



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