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 zoéas 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 zoéa 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 zoéa 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 zoéa, however, by making a single 
supposition, namely, that the ancestor from which the group 
of decapod has evolved had a larva like the zoéa, and that 
this larval form has been handed down to all of the de- 
scendants. 
The fate of the zoéa 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 
