Page:Popular Science Monthly Volume 22.djvu/386

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THE POPULAR SCIENCE MONTHLY.

traced to its adult form; and we have the converse of the case of entoconcha where the true affinities of a greatly modified adult are shown by its younger stages.

I give four figures to illustrate one such instance, which is not by any means exceptional or extreme; and in certain groups of animals, such as the Crustacea, such cases are quite numerous.

Fig. 5 is a magnified side-view of a crustacean, Sergestes, which is not quite full grown, but still essentially like the adult; and Fig. 6

PSM V22 D386 Sergestes larva.jpg
Fig. 7.

is a similar view of a closely related form, Lucifer, in a similar stage of development. Their close relationship is obvious at a glance, and their resemblances, which are much more conspicuous than their differences, are rendered more obvious by careful study; but the case is