70 Evolution and Adaptation 



full number of segments characteristic of the decapods, 

 new segments are intercalated between the cephalothorax 

 and abdomen. In fact, in many zoeas this intercalated 

 region is already in existence in a rudimentary condition, 

 and small appendages may even be present. A study of the 

 comparative anatomy of the crustaceans leaves no grounds 

 for supposing that the decapods with their twenty-one seg- 

 ments have been evolved from a thirteen-segmented form 

 like the zoea by the intercalation of eight segments in the 

 middle of the body. It follows, if this be admitted, and 

 it is generally admitted now, that the zoea does not repre- 

 sent an original ancestral form at all, but a highly modified 

 new form, as new, perhaps, as the group of decapods itself. 

 We are forced to conclude, then, that the presence of a larval 

 form throughout an entire group cannot be accepted as evi- 

 dence that it represents an ancestral stage. We can account 

 for the presence of the zoea, however, by making a single 

 supposition, namely, that the ancestor from which the group 

 of decapod has evolved had a larva like the zoea, and that 

 this larval form has been handed down to all of the de- 

 scendants. 



The fate of the zoea theory cast a shadow over the 

 nauplius theory, since the two rested on the same sort of 

 evidence. The outcome was, in fact, that the nauplius 

 theory was also abandoned, and this was seen to be the 

 more necessary, since a study of the internal anatomy of the 

 lowest group of crustaceans, the phyllopods, showed that they 

 have probably come directly from many segmented, annelid- 

 ian ancestors. The presence of the nauplius is now gener- 

 ally accounted for by supposing that it was a larval form 

 of the ancestor from which the group of crustaceans arose. 



The most extreme, and in many ways the most uncritical, 

 application of the recapitulation theory was that made by 

 Haeckel, more especially his attempt to reduce all the higher 

 animals to an ancestral double-walled sac with an opening 



