40 THE EVOLUTION OF THE METAZOA 



trochophore can occur only within the limits of an animal 

 group which is uniform both phyletically as well as from the 

 point of view of the comparative morphology. Yet in this 

 case it would be senseless to speak about a modified trocho- 

 phore. In my opinion we should dispense with special new 

 names for each different larval form and drop those that 

 have been already accepted. These names were given in former 

 days when specimens were found swimming in the sea plankton; 

 the puzzled scholars did not know where these animals 

 belonged and since each child must have its own name, 

 names were given to these specimens as if they were indepen- 

 dent organisms. It was later found that these specimens wxre 

 really larvae of some known or even unknown species. The 

 names, however, have been preserved together with their 

 suggestive influence, particularly in those cases where there 

 is some similarity between several larval forms which have 

 for this reason previously received a common name. 



As long as the name trochophore continues to be used it 

 should be Limited to the planktonic larvae of Annelida only. Lar- 

 vae of animal groups that stand either below or above Annelida 

 in the animal system should not be called trochophores even 

 in cases where they resemble the latter ; these similarities are 

 due first to analogous ways of Hfe which lead to convergences, 

 and, secondly, to an approximately equally low stage of 

 organization (the unsegmented stage). Even the orientation 

 of animals, e.g. the poised position in free water with its 

 longitudinal axis in the direction of the earth's centre of 

 gravity can lead to a general similarity in the habit of these 

 larvae. 



Certainly there won't be many objections if we do not call 

 the planktonic larvae of aproctous animals (Ameria) trocho- 

 phores, even if they look somewhat similar to the trochophore 

 of Polygordius which is considered as the most typical form 

 of all trochophores. These larvae are not numerous. Boas in 

 his excellent hehrhuch der Zoologie which was widely used in 

 his day, had already stated for good reasons that trochophores 



