It had also been found in one of the groups of higher crustaceans, the decapods, containing the crayfish, lobster, and crabs, that another characteristic larval form was repeated in many cases. This larva is known as the zoëa. It has a body made up of a fused head and thorax carrying seven pairs of appendages and of a segmented abdomen of six segments. The same kind of evidence that justified the formulation of the nauplius theory would lead us to infer that the zoëa is the ancestor of the decapods. The later development of the zoëa shows, however, that it cannot be such an ancestral form, for, in order to reach the full number of segments characteristic of the decapods, new segments are intercalated between the cephalothorax and abdomen. In fact, in many zoëas 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 segments have been evolved from a thirteen-segmented form like the zoëa 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 zoëa does not represent 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 evidence that it represents an ancestral stage. We can account for the presence of the zoëa, however, by making a single supposition, namely, that the ancestor from which the group of decapod has evolved had a larva like the zoëa, and that this larval form has been handed down to all of the descendants.
The fate of the zoëa 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, annelidian ancestors. The presence of the nauplius is now generally accounted for by supposing that it was a larval form of the ancestor from which the group of crustaceans arose.