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Chelicerates are in part defined by possessing chelicerate appendages, although crustaceans also possess chelate appendages. Chelicerates are more easily distinguished from other arthropods in lacking antennae and mandibles. Figure 4: Types of chelicerae: (A) jackknife, (B) scissor, and (C) 3-segmented chelate
In this malacostracan crustacean diagram, the maxillae are labelled maxilla and maxillula. In arthropods, the maxillae (singular maxilla) are paired structures present on the head as mouthparts in members of the clade Mandibulata, used for tasting and manipulating food.
Crustaceans have a pair of mandibles that typically consist of an enlarged basal segment (coxa) and a palp (sensory feeler) consisting of all other segments. In some groups, such as the Branchiopoda, the palp is reduced or absent. Crustacean mandibles may be equipped with special teeth (molar and incisor processes).
The decapod (crustaceans, such as a crab, lobster, shrimp or prawn) is made up of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (). [1] [2] Each segment may possess one pair of appendages, although in various groups these may be reduced or missing.
Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are a part of the subphylum Crustacea (/ k r ə ˈ s t eɪ ʃ ə /), a large, diverse group of mainly aquatic arthropods including decapods (shrimps, prawns, crabs, lobsters and crayfish), seed shrimp, branchiopods, fish lice, krill, remipedes, isopods ...
The labrum is a flap-like structure that lies immediately in front of the mouth in almost all extant Euarthropoda. The most conspicuous exceptions are the Pycnogonida, which are probably chelicerates. In entomology, the labrum amounts to the "upper lip" of an insect mouth, the corresponding "lower lip" being the labium.
Each maxilla consists of two parts, the proximal cardo (plural cardines), and distal stipes (plural stipites). At the apex of each stipes are two lobes, the inner lacinia and outer galea (plurals laciniae and galeae). At the outer margin, the typical galea is a cupped or scoop-like structure, located over the outer edge of the labium.
Mandibulates include the crustaceans, myriapods (centipedes and millipedes), and all true insects. The name "Mandibulata" refers to the mandibles , a modified pair of limbs used in food processing, the presence of which are characteristic of most members of the group.