The antennae cease to serve for locomotion,
their place is taken by the thoracic feet, furnished with long setae,
and by the long abdomen which just before was laboriously dragged along
as a useless burden, but now, with its powerful muscles, jerks the
animal through the water in a series of lively jumps. The anterior
antennae have lost their long setae, and by the side of the last
(fourth) joint, endowed with olfactory filaments, there appears a second
branch, which is at first of a single joint. The previously
multi-articulate outer branch of the posterior antennae has become a
simple lamella, the antennal scale of the Prawn; beside this appears the
stump-like rudiment of the flagellum, probably as a new formation, the
inner branch disappearing entirely. The five new pairs of feet are
biramose, the inner branch short and simple, the outer one longer,
annulated at the end, furnished with long setae, and kept, as in Mysis,
in constant whirling motion. The heart acquires new fissures, and
interior muscular trabeculae.
During the Mysis-period, the auditory organs in the basal joint of the
anterior antennae are formed; the inner branches of the first three
pairs of feet are developed into chelae and the two hinder pairs into
ambulatory feet; palpi sprout from the mandibles, branchiae on the
thorax, and natatory feet on the abdomen.
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