- This order of sessile-eyed decapods was absolutely unknown to science till 1779.
This modification is usually only found in the antennules and antennae, but it may exceptionally be found in the appendages of the trunk, as, for instance, in the thoracic legs of some Decapods (e.g.
A very curious function sometimes discharged by the antennules or antennae of Decapods is that of forming a respiratory siphon in sand-burrowing species.
In most Decapods the " stomach " or dilated portion of the fore-gut is divided into two chambers, a large anterior " cardiac " and a smaller posterior " pyloric."
The green gland and the structures associated with it in Decapods were at one time regarded as constituting an auditory apparatus.
True Decapods are first met with in Mesozoic rocks, the first to appear being the Penaeidea, a primitive group comprising the Penaeidae and Sergestidae, which occur in the Jurassic and perhaps in the Trias.