Copepoda
External morphology

The classification of the copepods is largely based on body tagmosis (division into functional regions) and segmentation and armature of the various limbs. The terminology adopted here for the description of the external morphology of copepods follows that of Huys and Boxshall (1991). The major body articulation subdivides the body into an anterior part - the prosome (Pr) - and a posterior part - the urosome (Ur) (Fig. Co.1a , Structure of Calanoid body).

The anterior region of the prosome, covered by the dorsal cephalic shield, is the cephalosome (Ce) which comprises the five cephalic somites bearing the antennules (A1), antennae (A2), mandibles (Md), maxillules (Mx1) and maxillae (Mx2), respectively, and the first thoracic segment bearing the maxillipeds (Mxp).

—Structure of Calanoid body and moutparts:

(Fig. Co.1a , Structure of Calanoid body)
(Fig. Co.1b , A1 antennule)
(Fig. Co.1c , A2 antenna)
(Fig. Co.1d , Md mandible)
(Fig. Co.1e , Mxp maxilliped)
(Fig. Co.1f , Mx1 maxillule)
(Fig. Co.1g , Mx2 maxilla)

In many copepods the second thoracic somite is also fused to the cephalosome, forming a cephalothorax. This somite bears the first pair of swimming legs (P1) and is called the first pedigerous somite (Pd1). There are seven postcephalic thoracic somites: the maxilliped-bearing somite, five pedigerous somites (Pd1-5), each bearing a pair of swimming legs (P1-P5) (Fig. Co.2), and the genital somite.

The seventh thoracic or genital somite (Gns) bears the sixth pair of legs (P6) which are highly modified and form the opercula that close off the paired female genital apertures (Huys and Boxshall, 1991).

Each pair of swimming legs is joined by an intercoxal sclerite which may be fused to the coxae. The swimming legs usually have two basal segments:
coxa (C) and basis (B). Two rami are articulated to the basis: an outer exopod (Exp) and inner endopod (Enp). The exopod and endopod are boardered by spines and/or setae which are denoted in the family descriptions according to the formula system of Sewell (1949) (Fig. Co.2).

— Comparison of the developmental pattern in podoplean and gymnoplean copepods (Fig. Co.3):

Within the Copepoda there are two major plans of body organization (tagmosis) into an anterior prosome and a posterior urosome, separated by the major body articulation (asterisks inFig. Co.3). In the gymnoplean tagmosis (Platycopioida and Calanoida) the major body articulation is located between the fifth pedigerous somite (Pd5) (= sixth thoracic somite), primitively bearing the P5, and the genital somite. In the podoplean type of tagmosis (all remaining copepod orders) the major articulation primitively lies between Pd4 and Pd5 (between fifth and sixth thoracic somites).

In gymnopleans, the urosome includes the anterior somite corresponding to the seventh thoracic somite (=genital somite). In females of Calanoida it is usually fused with the first abdominal somite, forming a genital double-somite.

The abdomen is the limbless postgenital region of the body. In podopleans the urosome comprises the sixth thoracic somite bearing the P5, the seventh thoracic somite and five abdominal somites. In the majority of Harpacticoida, Poecilostomatoida and Cyclopoida the female second and third urosomites (=seventh thoracic segment and first abdominal somites) are usually fused to form a genital double-somite. The difference between gymnopleans and podopleans may be observed even in the early copepodite development (Fig. Co.3). The last somite of the urosome is the anal somite bearing a pair of caudal rami (CR).

A detailed review of the literature on the copepod internal anatomy may be found in Blades-Eckelbarger (1986) and Brodsky et al., (1983). The original studies on the internal anatomy of Calanus finmarchicus and Epilabidocera amphitrites were made by Lowe (1935) and Park (1966), respectively. Comparative studies of copepod skeletomusculature were published by Boxshall (1985, 1990).