^^ ZOOGENESIS "^^^ 



Except for those just mentioned, all the animal phyla include 

 only forms which are bilaterally symmetrical. The echinoderms 

 appear to be radially symmetrical, but the radial symmetry is 

 far from perfect, and they are always bilateral in the young stages. 



In the preceding key the characters used in separating the 

 various phyla of bilaterally symmetrical animals are the most 

 obvious or the most easily understood, but not necessarily the 

 most important. 



It is generally agreed that the supporting rod known as the 

 notochord is a very important structural feature, so that the use 

 of its presence or absence in separating the Vertebrata, Cephalo- 

 chorda, Balanoglossida, Cephalodiscida and Tunicata from all 

 the other phyla can scarcely be questioned. 



The Vertebrata differ from the other four groups in having a 

 definite skull (they are therefore often called the Craniata), in 

 having the notochord surrounded by a stiff sheath and almost 

 invariably divided up into segments which correspond with those 

 of the embryonic muscular system, forming the "backbone," and 

 in almost invariably possessing a movable jaw and two pairs 

 of limbs. 



In the key they are paired with the Cephalochorda solely 

 because the latter in their general appearance resemble fishes far 

 more than they do anything else. But the Cephalochorda are 

 undoubtedly more closely related to the Tunicata, Balanoglossida 

 and Cephalodiscida — especially to the first named — than they 

 are to the Vertebrata. 



Regarding the groups from the Phoronidea to the Polyzoa 

 inclusive there is no agreement among zoologists as to what the 

 most important structural features are, or as to what the actual 

 interrelationships between the groups may be. 



t75] 



