General Features of the External Morphology of Marine Planktonic Copepods
The Maxillae (Maxilla) or second maxillae
They were incorrectly considered by various authors as anterior maxillipeds (Mxp1) due to their topographic proximity, and often their resemblance to the following appendages (fig. G3, E).
The sympodite is nearly normal, with the praecoxa and coxa most often fused, plus a basis. These foliose basal segments bear expansions (or endites) furnished with setae and an endopodite. The exopodite has disappeared.
In all gnathostome groups, the structure is comparable. The general tendency in all groups leads to a reduction in the number of endites (from the base towards the distal end) and a diversification of the setae, some of which are strong and spinulose, while others have a sensory function. Its shape and armature may be related to feeding regime (Landry & Fagerness, 1988; Ohtsuka, Shimozu, Tanimura et al., 1996).
The oral cavity, when it is broadly open, makes the assistance of the second maxilla useful for holding particles at the mouth.
In some forms of non-gnathostome Harpacticoids and Cyclopoids, the distal part of the appendage becomes more or less subcheliform and prehensile (fig. G3, E, b2). At the base of both maxillae, on the arthrodial membrane connecting the inner surface of the appendage to the cephalothorax, open the tips of the pair of maxillary glands. These represent the persistence of one of the excretory organs of an older segmental series, as shown during development by the disappearance of the antennal glands, which persist only in the small group Misophrioida (Boxshall, 1982).