The late Proterozoic or very earliest Cambrian was probably the time of the great splitting up into groups. The first development seems to have been among the trilobites themselves, the Hypoparia giving rise to two groups with compound eyes, first the Opisthoparia and later the Proparia. About this same time the Copepoda may have split off from the Hypoparia, continuing in the pelagic habitat. At first, most of the trilobites seem to have led a crawling existence, but about Middle Cambrian time they began to go back partially to the ancestral swimming habits, and retained some of the trunk segments to form a larger pygidium. The functional importance of the pygidium explains why it can not be used successfully in making major divisions in classification. Nearly related trilobites may be adapted to diverse methods of life. EVOLUTION WITHIN THE CRUSTACEA. The question naturally arises as to whether the higher Crustacea were derived from some one trilobite, or whether the different groups have been developed independently from different stocks. The opinion that all other crustaceans could have been derived from an _Apus_-like form has been rather generally held in recent years, but Carpenter (1903, p. 334) has shown that the leptostracan, _Nebalia_, is really a more primitive animal than _Apus_. He has pointed out that in Leptostraca the thorax bears eight pairs of simple limbs with lamelliform exopodites and segmented endopodites, while the abdomen of eight segments has six pairs of pleopods and a pair of furcal processes, so that only one segment is limbless. Contrasted with this are the crowded and complicated limbs of the anterior part of the trunk of _Apus_, and the appendage-less condition of the hinder portion. Further, a comparison between the appendages of the head of _Nebalia_ and those of _Apus_ shows that the former are the more primitive. The antennules of Nebalia are elongate, those of _Apus_ greatly reduced; the mandible of _Nebalia_ has a long endopodite, and Carpenter points out that from it either the malacostracan mandible with a reduced endopodite or the branchiopodan mandible with none could be derived, but that the former could not have arisen from the latter. The maxillæ of _Apus_ are also much the more specialized and reduced. _Nebalia_ being in all else more primitive than _Apus_, it follows that the numerous abdominal segments of the latter may well have arisen by the multiplication of an originally moderate number, and the last trace of primitiveness disappears. It is now possible to add to the results obtained from comparative morphology the testimony of palæontology, already outlined above, and since the two are in agreement, it must be admitted that the modern Branchiopoda are really highly specialized.