The spermatozoa are vibratile in Cirripedia: so too in Ostracoda after they have entered the female ducts3. In other Crustacea they are non-motile and are in Decapoda furnished with processes. They are generally united into spermatophores either by the secretion of the wall of the vasa deferentia or more rarely of special glands. The ova vary much in size, and in the quantity of secondary central yolk present. They are relatively largest when development is delayed or unaccompanied by metamorphosis. There is often a vitelline membrane and sometimes a specially secreted shell. The ova are rarely laid: they are attached by Cypris and Argulus to foreign bodies; deposited by the Stomatopoda in the burrows which they inhabit; left to float in a specially modified portion of the shell - the ephippium - as 'winter' ova by the Cladocera. They are generally carried about by the female, e. g. attached to the abdominal feet as in Decapoda; within the shell above the abdomen as in Cladocera; in a secreted ovisac, as by most Copepoda; or in a sub-thoracic brood-pouch by Arthrostraca.

1 The oviducts of Cirripedia open at the base of the first pair of limbs, the most anterior position known in Crustacea.

2 Candona is parthenogenetic in late autumn; so too Cypris vidua.

3 The sperm shows amoeboid motion in the Cladoceran Polyphemus, but not in other members of the order. See Zaccharias, Z. W. Z. xli. 1885. .

Development is rarely direct (Cladocera with the exception of Lepto-dora and certain Land-crabs), and in those instances where it is apparently direct or abbreviated (Arthrostraca), traces of metamorphosis are to be found, e.g. the larval membrane shed in ovo when the two pairs of antennae and the mandibles are formed in Oniscus and other Isopoda: or as soon as the blastoderm is established, as in Amphipoda. The metamorphosis may be well-marked but intra-ovular, e.g. Mysis, Nebalia. The majority of Entomostraca quit the egg as a Nauplius, an unseg-mented larva provided with three pairs of appendages, the first uniramose, the two others biramose, which correspond to the first and second antennae and mandibles respectively of the adult. The second pair often has a basal masticatory hook, and is innervated from a post-oral ganglion. There is a median eye. Somites and appendages are formed by subsequent growth accompanied by ecdyses. Among Malacostraca the Schizopod Euphausia and Decapod Penaeus have a Nauplius-stage. The latter passes through a Zoaea- and a Mysis-stage into the adult.

The Decapod Lucifer starts as a Meta-Nauplius with the rudiments of the two pairs of maxillae and the first maxillipeds, in addition to the Nauplius appendages: its ally Sergestes as a Protozoaea, in which all these limbs are well-developed, and a cephalothoracic shield and unsegmented abdomen are present as well. Most other Decapoda are hatched in a later stage known as Zoaea, in which the second and third maxillipeds and a segmented abdomen devoid of limbs are present. The thorax is short, not segmented, but contains a ganglionic mass pierced by the sternal artery. There is an azygos Nauplius-eye in addition to the stalked paired eyes. The Zoaea like the Nauplius acquires secondary characters. Many Decapoda pass through a Mysis-sts.ge, i. e. with biramose natatory feet on the thorax, a stage in which the lobster (Homarus) is hatched. Development is still more abbreviated and becomes almost direct in some forms, e.g. Astacus. The larva of the Stomatopoda is known as Alima and Erichthus; of the Palinuridae (Decapoda) as Phyllosoma. The true Crabs (Decapoda Brachyura) pass through a stage, Megalopa, resembling certain Hermit crabs.

A few Amphipoda, Isopoda, and Decapoda are terrestrial. The Branchio-poda are exclusively found in fresh water or in brine: one or two Cirri-pedia occur in brackish waters. Other Cirripedia, like the Leptostraca, Cumacea, and Stomatopoda, are exclusively marine. Representatives of other orders are both marine and fresh water: the majority of Schizopoda and Decapoda however are marine1. Several Cirripedia, many Copepoda, and some Isopoda are parasitic. Polymorphism has been observed among male Artkrostraca, principally in Amphipoda. A development of colour in connection with reproductive activity is seen in some Cladocera. Phosphorescence occurs among marine Copepoda (e. g. Sapphirina), Ostracoda, and in the Euphansidae among Schizopoda. The majority prey upon animals or plants. The genus Estheria is found in the Devonian strata: other supposed Phyllopoda, e. g. Hymenocaris, from Silurian and Carboniferous strata, may be allied to Leptostraca. The shells of Ostracoda occur in most formations. Cirripedia date from the Inferior oolite: Artkrostraca from Devonian strata: Decapoda from the Coal measures.