Crab
Cancer crabs are a group of marine crustaceans of the family Inachidae comprising about 40 species. They have been called "true crabs" to distinguish them from related groups such as king crabs and porcelain crabs, which represent convergent evolution rather than true relationship.
Cancer crabs or True Crabs (Brachyura) are dorsoventrally (top to bottom) flattened crabs with an oval outline. Most species are covered with a thick exoskeleton, composed primarily of highly mineralized chitin,] and armed with a pair of chelae (claws). Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 4 m (13 ft).[6] Several other groups of crustaceans with similar appearances – such as king crabs and porcelain crabs are not true crabs but have evolved features similar to true crabs through a process known as carcinization.
Cancer crabs are sometimes called "true crabs", to distinguish them from king crabs and porcelain crabs, which represent convergent evolution rather than a true relationship. They have an oval outline. Most species are covered with a thick exoskeleton, composed primarily of highly mineralized chitin], and armed with a pair of chelae (claws). Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 4 m (13 ft). Several other groups of crustaceans with similar appearances – such as king crabs and porcelain crabs are not true crabs but have evolved features similar to true crabs through a process known as carcinization.
Cancer crabs are sometimes called male and female, but most species have no distinct morphological differences between the sexes. The male reproductive organs include an anterior pair of testes; a pair of vasa deferentia, seminal vesicles, and a median infundibulum (spermatophore-receiving cavity). The female organs include a single ovary and oviduct, often with an albumen gland. The nauplius larvae that hatch from fertilized eggs do not resemble the adults; they go through several larval stages before reaching maturity, and in many species change anatomical features many times in their lifetimes.