THE NEW GENEALOGICAL TREE 451 



This may be the best place to call attention to a fact which 

 has been, it seems to us, too much neglected till now. It is 

 well known that a planktonic larva appears during the onto- 

 geny of the Enteropneusta which is typical of these animals; 

 this is the so-called tornaria, one of those larvae that had 

 been developed by the Echinodermata and which have been 

 given various names. Zoologists have therefore considered 

 that there is a probability of a closer relationship between these 

 two groups of the Oligomeria. All these larvae, as w^ell as the 

 various planktonic larvae of the Ameria and the Polymeria 

 belong to the category of the primary larvae, as has already 

 been emphasized, i.e. they appear in a form with a lower 

 level of organization. Very frequently we can find the phase 

 of these larval forms transmuted from a freely living phase 

 of the development of an animal and incorporated into an 

 embryonic phase of the ontogeny. It should be sufficient if we 

 call attention here to conditions that can be observed in 

 those crustaceans that Hve in fresh water. 



This tornaria can now be identified as the last primary 

 planktonic form of its kind. The institution of a primary 

 planktonic larva had been abandoned during the comparatively 

 short phase of the phylogenetic evolution w^hen the first 

 chordate evolved from its oligomerous ancestor. It was chang- 

 ed into an embryonic stage, a development similar to that 

 of the nauplius in the evolution of the crayfish (Potamobius), 

 among the decapod crustaceans. It seems easy to attribute this 

 change to a transition to a life in the fresh water; it is con- 

 sidered that a reduction of a freely living larval stage regularly 

 takes place when the animal relinquishes its Hfe in the 

 sea. 



There are, however, some exception where this rule does 

 not seem to be valid (e.g. in connection with the Dreissensia 

 polymorpha, a species of bivalve molluscs which had probably 

 penetrated late into the fresh water, and with the phylac- 

 tolaematous Ectoprocta with their considerably changed larvae 

 that develop forms similar to cormi). 



