Crustaceans (Crustacea Brünnich, 1772) constitute a subphylum of the Arthropods that includes primarily marine aquatic animals, although they are also widely found in fresh waters and a few terrestrial species are known. Thanks to various molecular studies, it is now well accepted that the crustacean group is paraphyletic and includes all animals in the clade pancrustacea except the hexapods. Some crustaceans are therefore more similar to insects and other hexapods than other crustaceans.

The Crustacea is a very heterogeneous group whose members, at morphological level, are united mainly by two basic characters:

  • the presence of two pairs of preoral appendages (antennules and antennae) in the cephalon (head), otherwise identical – in number and arrangement of segments and appendages – to that of Myriapods and Hexapods;
  • the presence of some biramous appendages (different, however, from those of Trilobitomorphs).

In the most primitive Crustacea there are still signs of homonomous metamerism in the trunk, the only region after the cephalon: in this case all segments of the trunk have locomotor appendages, which usually become smaller approaching the last segment, which often has a rigid fork.

In the most evolved crustaceans, the trunk is divided into thorax and abdomen, called respectively pereion and pleon. In this case there is a differentiation of appendages in pereiopods (in pereion) and pleiopods (in pleon): pereiopods are mainly used for terrestrial locomotion (the most anterior pereiopods can perform the function of taking and shredding food and in this case are called massillipeds), while the pleiopods perform other functions (for example they can be shaped like a paddle for swimming, can carry gills, serve to hold eggs).

Leave a Comment