After this brief survey of Crustacean organisation we may be able to form an opinion upon the position of the Crustacea relative to other Arthropoda, and upon the question debated some time ago in the pages of _Natural Science_[13] whether the Arthropoda constitute a natural group. The Crustacea plainly agree with all the other Arthropoda in the possession of a rigid exoskeleton segmented into a number of somites, in the possession of jointed appendages metamerically repeated, some of which are modified to act as jaws; they further agree in the general correspondence of the number of segments of which the body is primitively composed; the condition of the body-cavity or haemocoel is also similar in the adult state. An apparently fundamental difference is found in the entire absence during development of a segmented coelom, but since this organ breaks down and is much reduced in all adult Arthropods, it is not difficult to believe that its actual formation in the embryo as a distinct structure might have been secondarily suppressed in Crustacea. The method of breathing by gills is paralleled by the respiratory structures found in _Limulus_ and Scorpions; the transition, if it occurred, from branchiae to tracheae cannot, it is true, be traced, but the separation of Arthropods into phyletically distinct groups of Tracheata and Branchiata on this single characteristic is inadmissible. On the whole the Crustacea may be considered as Arthropods whose progenitors are to be sought for among the Trilobita, from whose near relations also probably sprang _Limulus_ and the Arachnids. CHAPTER II CRUSTACEA (_CONTINUED_): ENTOMOSTRACA—BRANCHIOPODA—PHYLLOPODA—CLADOCERA— WATER-FLEAS SUB-CLASS I.—ENTOMOSTRACA. The Entomostraca are mostly small Crustacea in which the segmentation of the body behind the head is very variable, both in regard to the number of segments and the kind of differentiation exhibited by those segments and their appendages. An unpaired simple eye, known as the Nauplius eye from its universal presence in that larval form, often persists in the adult, and though lateral compound eyes may be present they are rarely borne on movable stalks. In the adult the excretory gland (“shell-gland”) opens on the second maxillary segment, but in the larval state or early stages of development a second antennary gland may also be present, which disappears in the adult. The liver usually points forwards, and is simple and saccular in structure, and the stomach is not complicated by the formation of a gastric mill. With the exception of most Cladocera and Ostracoda the young hatch out in the Nauplius state. =Order I. Branchiopoda.=[14]