358 ^^^ ANIMAL KINGDOM 



Mouth- 



Esopha^us" 



Laleral 



.,/— Ovary 



-Anixs 



"Testis 



"Tail fin. 



Figure 18.5. Chaetognatha. A ventral view of a mature specimen. (Modified from 

 Parker and Haswell.) 



gist, who is interested in any animal that presents a unique way of life. 

 These phyla represent life forms that have failed to dominate the scene, 

 and this in itself is a challenging problem. They are of special interest 

 to the student of phylogeny, for among them may be found intermediate 

 stages that will reveal how the major groups arose. It is evident from 

 the foregoing (and from the discussions of minor phyla in other chapters) 

 that in some cases a study of minor groups has helped our understanding 

 of phylogeny. In other cases new and interesting situations are revealed 

 that are of little use in the understanding of other groups, and in some 

 instances the result is more confusion rather than less. Although they 

 offer no simple solution to the problem of phylogeny, the minor phyla 

 enrich the subject considerably. 



