Plaie XXII. 



(Left) Two views of the 'trochophore' of the worm Pol}[^ordiHs lactens: top, from above; bottom, 



side view; the mouth in each is to the right. 



[Rij^ht) Segmenting stage of the worm Autolytus. 



Photographs by D. P. Wilson 



swimming nauplius in some prawns (Pcnacidca). The young decapod hatches 

 at a later stage, typically a 'zoca' as in the crabs (Fig. 21 ; 8: Plates XVIII and 

 XXIV). There then follows a series of moults, often a long series, each a little 

 further advanced than the previous one, before the adult stage is reached. 

 There may be quite distinct changes in form as in crabs, for example, where 

 the zoea changes to a 'megalopa' (Fig. 21 ; 9: Plate XXIV), or the change may 

 be much more gradual as in shrimps. A series of examples of decapod larvae 

 is illustrated in the figure. Some are very exotic in style such as the spiny 

 larva of the oceanic prawn Sergestes (Fig. 22; 6) and the flat 'phyllosoma' (Fig. 

 21; 12) of the spiny lobster which is the French 'langouste'. These queer 

 transparent larvae can live in the plankton for some six months and be carried 

 T,ooo miles before going to the bottom to become little spiny lobsters. It is 

 this ability that is responsible for the French fishery for 'langouste' in the deep 

 water west of the Hebrides and for the less abundant spiny lobster stocks in 



