In the seas, bony fish and crab-like decapods increased in numbers and variety, while pelecypods and gasteropods took the prominent place previously occupied by ammonites and belemnites, and, leaving behind such forms as Rudistes, Inoceramus, &c., they gradually developed in the direction of the modern regional groups.
- This order of sessile-eyed decapods was absolutely unknown to science till 1779.
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.
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.