THE P EMEUS. 



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 zo'ca two pairs of fissures and an interior 

 apparatus are always recognisable. 



During the next zoeal 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. Miiller). 



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 zoea passes into the " mysis 

 form." The antennae cease to serve for locomotion, their place is 

 taken by the thoracic feet, furnished with long hair-like spines 

 (setae), and by the abdomen, which just before was laboriously 

 dragged along as a useless burden, but now, with its powerful 



