The present volume does not contain a key to species of North Sea copepods.
Copepoda are crustaceans without a carapace; the cephalon, however, has a well developed cephalic shield. There is a single, median, simple maxillopodan eye (sometimes absent). One or more thoracomeres are fused to the head to form a cephalosome. The thorax consists of six segments, the first is fused to the head and bears maxillipeds. There are four to five pairs of natatory thoracopods, the posterior thoracopods are always biramous. The abdomen consists of five segments including the anal somite (= telson). The caudal rami are well developed; the abdomen is without appendages, except for an occasional reduced pair on the first segment associated with the gonopores. The antennules are uniramous, the antennae are uni- or biramous.
The copepod body is generally divided into three distinct tagmata (or regions): 1) the cephalon or cephalosome, bearing the usual head appendages and a pair of maxillipeds; 2) the thorax or metasome, bearing al the other limbs; 3) abdomen or urosome, without any limbs.
The majority of the free-living copepods belong to the Order Calanoida, Order Harpacticoida and Order Cyclopoida. All these orders have representatives in the plankton of the North Sea.