If the adductor muscles holding the carapace valve together are sliced through, one of the carapace valves can be folded back to reveal the arrangement of the limbs.
Second antennae: The largest limbs are the second antennae; the muscular protopodites of each second antenna, which generate the power for swimming occupy between a third to half of the length of the carapace. The first exopodite segment, which is slim and usually about half the length of the protopodite, projects laterally out of the carapace through the incisure. In many species there is a short seta that curls around the suture between the first and second podomeres, which does not appear to have been noticed previously. There are a further eight exopodite segments, which each carrying a long setae feathered with secondary filaments; the terminal segment carries one or two short unfeathered setae. The setae and their filaments fan out during the propulsive stroke of the exopodite and feather during the recovery stroke. The terminal exopodite segment usually carries one or two additional short setae. On the inner surface of the protopodite is an endopodite. These endopodites are sexually dimorphic. In females it is composed of two segments, the first can have an anterior process called the processus mamillaris whose presence or absence is a valuable taxonomic character for differentiating between the families. The terminal segment carries five setae, the three shorter ones are thin walled and are probably sensory. In males there is a third segment, which is developed into a hook appendage. The two longest endopodite setae are carried on the second segment, and the three subequal thin-walled sensory setae are inserted on the basal hasp of the hook. The hook appendage (or clasping organ) on the left limb is larger and more elaborately developed than the appendage on the right limb, which in some species is reduced to a straight process (e.g. Obtusoecia spp).
First antennae and frontal organs: Between the basales of the second antennae are the two first antennae, and between them is the frontal organ. Both the first antennae and the frontal organ are sexually dimorphic. In males the stem of the frontal organ is anchored to the second segment of the antenna by a dorsal seta from the second segment that curls around it; in females these dorsal setae if present are straight and project forward. There is always a clear suture between the stem and the capitulum of the frontal organ in males, whereas in several genera the stem and capitulum of the frontal organ of females are fused and may be undifferentiated. During swimming the terminal setae of the first antennae are held splayed out in front of the animal. In males the limb segments of the first antennae are always clearly separated by sutures, but in the females of several genera the limb segments are fused. In males three of the terminal setae are long, thick-walled and slender and the terminal seta (the e seta), which is usually the longest, carries an armature of spines. The characteristics of this armature (numbers and arrangement of the spines) are important taxonomic characters. In females the four of the five terminal setae are thin-walled and sensory, and only one is much longer than the others and is slender and thick-walled. In some genera, notably in Mikroconchoecia, the sensory setae are bifid.
Mandibles: Posterior to the second antennae and flanking the mouth are a pair of mandibles, with well-developed endopodites that are used to manipulate food items at the mouth. The basal segments are developed into an elaborate system of tooth rows, which are also useful in identification. The mandibles show no sexual dimorphism. The exopodites are generally reduced to small pegs on the basale that carry a long plumose seta, but in Pseudoconchoecia the endopodite is enlarged, thin walled and flattened and lacks a seta. In some of the Bathyconchoecia species there are up to three setae.
Maxillae: The next pair of limbs are the maxillae. These show minor differences in the numbers of setae between the various genera and are not used in species identification. The exopodites manipulate food particles at the mouth.
Fifth pair: Manipulate food particles between the gape of the carapace valves. They show little sexual dimorphism and have not to date been used in identification. However, their setation, and in particular the relative lengths of the terminal claw setae, vary between genera, and may prove useful in defining the genera.
Sixth pair: Are strongly sexually dimorphic. In males the limb is strongly developed and muscular and its three terminal setae are generally long and filamentous and in most species subequal. They generally carry long secondary hairs, but in those species in which the lengths of these terminal setae are markedly disparate, these secondary hairs can be lacking. It has been suggested that the male glands situated just below the posterior dorsal corner release secretions onto the secondary hairs and play some role in mating. In females the terminal setae are unequal, and claw-like and help to manipulate food in the gape between the carapace valves. Once again variations in their relative lengths may prove useful character in defining genera. Both the fifth and sixth limbs have lateral plates (epipodial appendages) that carry three groups of between 4 and 7 long setae, sometimes referred to as the ‘vibratory plates’. In live animals these setae can be seen beating continuously and drive the respiratory flow of water through the carapace.
Seventh pair: These limbs are vestigial in halocyprids and are reduced to two segments with the terminal segment carrying two unequal setae. In contrast, these limbs are multi-segmented in the myodocopids and carry an elaborate array of bristles. They continuously writhe around over the surface of the abdomen and so possibly play a role in keeping the inner surfaces of the carapace clean.
Caudal furca consists of two flanges which are armed with 8 pairs of hook setae. The longest first pair is off-set from the others, which progressively diminish in size posteriorly. The number of these caudal spines increases with each developmental ecdysis. The earliest juvenile instars have just two pairs of hook setae and an additional pair is added at each moult. So at maturation, after the six ecdyses, the adult number of eight pairs of claw setae is reached. However, in some of the Archiconchoecini species adults have fewer pairs of claw setae, and the suggestion is that they maturity neotenously after fewer juvenile ecdyses. Another curious difference between the halocyprids and the myodocopids is that the anus opens in the fold just anterior to the base of the furca in halocyprids, but opens between the furcal lamellae in myodocopids.
Copulatory appendage: As note above in male halocyprids the appendage is a single structure located on the right side of the caudal furca, and this asymmetry is probably related to the asymmetries seen in the openings of the carapace glands and the hook appendages on the endopodites of the male second antennal. In myodocopids, the copulatory appendage is a paired structure that is place symmetrically at the base of the furca, and there is no sign of any asymmetries in the structure of the limbs and carapace.