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.