Now, with regard to the segmentation of the body, the Crustacea fall into three categories: the Entomostraca, in which the number of segments is indefinite; the Malacostraca, in which we may count nineteen segments, exclusive of the terminal piece or telson and omitting the lateral eyes; and the Leptostraca, including the single recent genus _Nebalia_, in which the segmentation of head and thorax agrees exactly with that of the Malacostraca, but in the abdomen there are two additional segments. It has been usually held that the indefinite number of segments characteristic of the Entomostraca, and especially the indefinitely large number of segments characteristic of such Phyllopods as _Apus_, preserves the ancestral condition from which the definite number found in the Malacostraca has been derived; but recently it has been clearly pointed out by Professor Carpenter[3] that the number of segments found in the Malacostraca and Leptostraca corresponds with extraordinary exactitude to the number determined as typical in all the other orders of Arthropoda. This remarkable correspondence (it can hardly be coincidence) seems to point to a common Arthropodan plan of segmentation, lying at the very root of the phyletic tree; and if this is so, we are forced to the conclusion that the Malacostraca have retained the primitive type of segmentation in far greater perfection than the Entomostraca, in some of which many segments have been added, _e.g._ Phyllopoda, while in others segments have been suppressed, _e.g._ Cladocera, Ostracoda. It may be objected to this view of the primitive condition of segmentation in the Crustacea that the Trilobites, which for various reasons are regarded as related to the ancestral Crustaceans, exhibit an indefinite and often very high number of segments; but, as Professor Carpenter has pointed out, the oldest and most primitive of Trilobites, such as _Olenellus_, possessed few segments which increase as we pass from Cambrian to Carboniferous genera. The following table shows the segmentation of the body in the Malacostraca, as compared with that of _Limulus_ (cf. p. 263), Insecta, the primitive Myriapod _Scolopendrella_, and _Peripatus_. It will be seen that the correspondence, though not exact, is very close, especially in the first four columns, the number of segments in _Peripatus_ being very variable in the different species.