THE PENEUS. 459 
slightly in locomotion. The forked tail is remarkable, and is 
unlike the flat apparatus of many other Crustacea. The heart 
possesses only one pair of fissures, and has no muscles traversing 
its interior, whilst in other zoéa two pairs of fissures and an interior 
apparatus are always recognisable. 
During the next zoéal period the paired eyes, the segments of 
the middle body and abdomen, the hind foot jaws, the sides of the 
YOUNG ZOEA OF THE SAME PRAWN. 
Magnified 45 diameters (after F. Muller). 
tail, and the stump-like rudiments of the feet of the middle body 
are formed. 
The appendages of the tail-piece sprout forth like other limbs 
freely on the ventral surface. 
As the feet of the middle body come into action simultaneous 
with other profound changes, the zoéa passes into the ‘“ mysis 
form.” The antenne cease to serve for locomotion, their place is 
taken by the thoracic feet, furnished with long hair-like spines 
(seta), and by the abdomen, which just before was laboriously 
dragged along as a useless burden, but now, with its powerful 
