Their appearance in the air-breathing Arachnids does not separate those forms from the water-breathing Arachnids which are devoid of them, any more than does their appearance in certain Amphipoda separate those Crustaceans from the other members of the class.
Orders: Heterobranchia (Branchiopoda, Isopoda, Amphipoda, Stomapoda), Homobranchia (Decapoda).
Orders: (a) Malacostraca: Decapoda, Stomapoda, Amphipoda, Laemodipoda, Isopoda; (b) Entomostraca: Branchioloda, Poecilopoda, Trilobitae.
Orders: Amphipoda, Loemodipoda and Isopoda.
- As in the genuine Isopoda, the eyes of Amphipoda are always sessile, and generally paired, and, in contrast to crabs and lobsters, these two groups have only four pairs of mouthorgans instead of six, but seven pairs of trunk-legs instead of five.
Many of the Amphipoda are extremely voracious.
(Amphipoda), Sars (1890-1895), demanding attention, and the quite recent Amphipoda of the Hirondelle, Chevreux (1900), and Hyperiidea of the Plankton-Expedition, Vosseler (1901).
In the Decapoda three pairs are thus modified, and in the Tanaidacea, Isopoda and Amphipoda only one.
In the Amphipoda, the gills though arising from the inner side of the bases of the thoracic legs are probably also epipodial in nature.
In the other Malacostraca the heart is generally abbreviated, and even where, as in the Amphipoda, it is elongated and tubular, the ostia are restricted in number, three pairs only being usually present.
In some Amphipoda the secretion of glands on the body and limbs is used in the construction of tubular cases in which the animals live.
In certain Copepoda and Ostracoda glands of the same type produce a phosphorescent substance, and others, in certain Amphipoda and Branchiura, are believed to have a poisonous function.
Apart from certain doubtful and possibly abnormal instances among Phyllopoda and Amphipoda, the only exceptions are the sessile Cirripedia and some parasitic Isopoda (Cymothoidae), where hermaphroditism is the rule.
A few Isopoda are known from Secondary rocks, but their systematic position is doubtful and they throw no light on the evolution of the group. The Amphipoda are not definitely known to occur till Tertiary times.