126 ELEMENTS OF BIOLOGY 



really connotes that the various phyla have appeared in the order 

 given, but the historical origins are in most cases by no means clear. 

 As an example of procedure in establishing the position of an ani- 

 mal the simplest possible method has been given, namely, comparing 

 the external features of the adult house fly with those of other ani- 

 mals and classifying it according to similarities and dissimilarities. 

 But the specialist has still other criteria that show relations between 

 animals. Regardless of the apparent differences in the structure of 

 adult animals, it is considered that animals which pass through the 



PROTOTROCH 



MOUTH 



MOUTH / Jte,.; ^^^. J^..,^.;^^= — METATROCH 



PROTONEPHRIDIUM 



SHELL 

 -ANUS 



Fig. 76. — Two types of trocophore larvx. Compare the trocophore of an annelid 

 •on the left with that of a mollusc on the right. (After Kiihn: Griindnss der allge- 

 meinen Zoologie. Georg Theime, Leipzig.) 



same or like stages during their development from the egg are re- 

 lated. Thus many Annelida, MoUusca, some MoUuscoidea, and a 

 worm-like form of uncertain position (Gephyrea) all pass through 

 a larval stage, called a trochophore (Fig. 76), that is very similar; 

 these phyla consequently may be regarded as more or less re- 

 motely related. Likewise, all Vertebrata pass through very similar 

 stages and develop in much the same fashion. Then, too, fossil re- 

 mains of animals frequently indicate that some of the modern ani- 

 mals are related through common ancestors. Consequently, technical 

 classification is based upon three criteria, comparison of the adult 

 forms, developmental history, and relation to fossil types. 



