In the Decapoda the branchial system is more complex.
It is among the Malacostraca, however, and especially in the Decapoda, that the " gastric mill " reaches its greatest perfection.
In the Decapoda three pairs are thus modified, and in the Tanaidacea, Isopoda and Amphipoda only one.
In the Decapoda the antennal gland is largely developed and is known as the " green gland."
A minute entosternite having the above-described structure is found in the Crustacean Apus between the bases of the mandibles, and also in the Decapoda in a similar position, but in no Crustacean does it attain to any size or importance.
Orders: Decapoda and Stomopoda.
In the Decapoda, where the antennal gland alone is well-developed in the adult, the maxillary gland sometimes precedes it in the larva.
In the higher Decapoda, numerous additional centres are developed in the brain and its structure becomes extremely complex.
Orders: (a) Malacostraca: Decapoda, Stomapoda, Amphipoda, Laemodipoda, Isopoda; (b) Entomostraca: Branchioloda, Poecilopoda, Trilobitae.
In the Malacostraca it is absent in the adult, or persists only in a vestigial condition, as in some Decapoda and Schizopoda.
It is most typically developed in the most specialized Decapoda, the Brachyura, while the more primitive groups of Malacostraca, the Euphausiidae, Penaeidea and Stomatopoda, retain the primitive order of appearance C FIG.
In Palaeozoic formations, from the Upper Devonian onwards, numbers of shrimp-like forms are found which have been referred to the Schizopoda and the Decapoda, but here again the scanty information which may be gleaned as to the structure of the limbs rarely permits of definite conclusions as to their affinities.
In the majority of the Decapoda there is a saccular invagination of the integument in the basal segment of the antennular peduncle having on its inner surface " auditory " setae of the type just described.
- Many Crustacea belonging to very different groups (Ostracoda, Copepoda, Schizopoda, Decapoda) possess the power of emitting light.
In the higher Decapoda the male is generally larger than the female and has stronger chelae.