90 Mutations and Evolution. 



Zoologists are generally agreed that the nauplius and zoaea 

 larvse of Crustaceans and the veliger larva of Molluscs represent 

 ancestral stages ; similarly, that the trochophore larva of Annelids 

 and Molluscs shows these two very divergent groups to have been 

 derived from a common ancestor. MacBride would go further and 

 say that the trochophore is a smaller and somewhat reduced or 

 specialized representative of that ancestor. According to the 

 interpretation here adopted, the various larvae at one time 

 represented terminal stages in their respective life-cycles, and by 

 subsequent evolution they were not modified out of existence or out 

 of recognition through germinal changes, but became subterminal 

 through the addition of later stages to complete the ontogeny. In 

 a field where every larva has its ancestral significance, it is almost 

 invidious to single out particular cases for illustration. The tadpole 

 of Ascidians, which gave the first hint of the Chordate affinities of 

 this group, is a striking instance. Only the degeneration and special 

 adaptations resulting from the adoption of a sedentary life can 

 account for their remarkable transformation, and we fail to see how 

 it can be adequately explained except on a neo-Lamarckian basis. 

 An equally irrefutable case of recapitulation is that of the feather- 

 star Antedon, whose egg developes into a free-swimming larva which 

 later becomes fixed by a stalk. This fixed stage corresponds with 

 the modern genus Pentacrinus. The adult feather-star developes 

 cirri and, losing its stalk, becomes free again. Can anyone pretend 

 to believe that this remarkable series of transformations does not 

 recapitulate the history of the race ? And, if so, how can one avoid 

 the conclusion that the life-cycle has been lengthened? 



A type of recapitulatory character corresponding to the 

 appearance of evanescent stages in the gametophytes of the 

 Gymnosperms, is found in certain nauplius larvae. MacBride (I.e. 

 p. 204) regards the nauplius as representing the common ancestor 

 of all Crustacea. He says, " When the larva does not hatch out as 

 a nauplius, a cuticle is produced and shed by the embryo whilst 

 still within the egg shell when it reaches the nauplius stage, thereby 

 showing that formerly this stage must have been passed through in 

 the open, in the ancestors of the forms in which it is now purely 

 embryonic." Again, in the oyster, which has no foot and becomes 

 permanently attached by one valve of its shell, the veliger larva 

 nevertheless possesses a functional foot which is therefore a 

 recapitulatory character. 



