THE EVIDENCE FROM DEVELOPMENT. 



215 



division of the prawn tribe which includes the species of Penceus 

 (Fig. 132) as its representatives. 



The prawns, as everyone knows, are intimately associated with 

 the lobsters, shrimps, and crayfish 

 as higher Crustacea. Yet the first 

 appearance of Penseus is not, as in 

 the crayfish, as a well-nigh per- 

 fectly-formed animal, nor, as in the 

 lobster (Fig. 129), as a Zoe'a, some- 

 what like the adult ; nor yet, as in 

 the crab, as a Zoe'a (Fig. 130 a), 

 widely different from the mature 

 form. On the contrary, the 

 youngest stage of Penaeus is a 

 veritable "Nauplius" (Fig. 133), 

 with three pairs of appendages, 

 and a single median eye, accu- 

 rately reproducing the features 

 now familiar to us in the Barnacle FlG - W-NAUPLIUS OF PEK^US. 

 (Fig. 119), Sacculina (Fig. 121), and lower crustacean life (Fig. 123,6) 

 at large. Next in order, this Nauplius develops a rounded body-shield 

 (or carapace); the first and second 

 pairs of appendages becoming the 

 two pairs of feelers proper to all crusta- 

 ceans, whilst the third pair becomes 

 the chief jaws or "mandibles." Then 

 are developed four pairs of feet, con- 

 verted in due time into jaws and 

 foot-jaws ; and behind these appear 

 other five pairs of appendages which 

 become the ten walking feet. The 

 six joints of the tail have as yet no 

 appendages, but the tail itself ends in 

 two tufted processes, and we see the 

 Zoea-form (Fig. 134) thus limned out; 

 whilst no less remarkable is the resem- 

 blance of the young prawn at this stage 

 to an adult Cyclops (Fig. 116, A) water- 

 flea. Two stalked eyes, in addition to 

 the single eye of the Nauplius, appear 

 in the Zoea-form, which alters and 

 changes through the decrease of the 

 feelers, till now used for swimming. 



The tail now increases in size and replaces the feelers in function ; 

 and the feelers, each at first double, become single- jointed organs. 

 The five feet of the chest-region are each provided with two terminal 



FIG. 134. ZOEA OF 



