The Origin of the Crustacea *

Pre‐Cambrian metamerically segmented bilaterians that ultimately gave rise to crustaceans probably arose from unsegmented flatworms. The recent suggestion that early arthropods, far from possessing a capacious segmented coelome of the annelid type, may never have had such, is attractive. Crustaceans were probably derived from small, segmented, surface‐dwelling non‐annelidan marine worms with a haemocoele. Their appendages probably originated as simple outgrowths whose shape was maintained by haemocoelic pressure. Possible routes whereby trunk limbs could have been derived from such rudiments are suggested. Trunk limbs would originally be unsegmented, as in many extant branchiopods and in certain Cambrian crustaceans. The evolution of thoracopodal feeding and some of the factors involved in the differentiation of the cephalic appendages are considered, as is the origin of the nauplius larva and the establishment of its feeding mechanism. Certain features of the cephalic region of the adult reflect changes necessitated as a result of the incorporation of the nauplius into the life cycle. Ontogeny would originally be anamorphic and follow the pattern preserved in its most primitive form in certain extant anostracan branchiopods. A reconstruction of the Ur‐crustacean is attempted. Justification for features not previously associated with such a reconstruction, such as locomotory antennae, a relatively short trunk with only a short series of limbs and a limbless posterior region, and unsegmented trunk limbs, is provided by fossil evidence, functional considerations and the situation in primitive extant forms. Crustaceans were evidently not derived from any known arthropod clade. Stem lineage forms probably arose from the same group of pre‐crustacean ancestors. While the Crustacea appears to be a monophyletic group, the idea that arthropodization must have occurred more than once and that the Arthropoda is a polyphyletic assemblage is supported, and evidence in favour of this view is cited.