THE METAMORPHOSES OF CRUSTACEA 79 



as examples of this mode of development. The 

 Woodlice and their immediate allies differ a little 

 from the other members of the division in the fact 

 that the young leave the brood-pouch with the last 

 pair of legs still undeveloped, though in other 

 respects they are like miniature adults. 



In those Crustacea which have a direct develop- 

 ment without free-swimming larval stages, it is 

 sometimes possible to find traces of such stages in 

 the early development of the embryo. This is 

 shown most clearly, perhaps, in the Opossum 

 Shrimps (Mysidacea). In these the embryo be- 

 comes free from the egg-membrane (or may, in a 

 sense, be said to " hatch ") at a very early stage, and 

 lies free within the brood-pouch as a maggot-shaped 

 body, on which three pairs of rudimentary limbs can 

 be made out. The later development shows that 

 these three rudiments correspond to the antennules, 

 antennae, and mandibles, so that the maggot-shaped 

 embryo is, in fact, a disguised nauplius without the 

 power of swimming or of leading an independent 

 existence. In other cases as, for instance, in the 

 Crayfish, where the earlier stages are confined 

 within the egg-membrane (or " egg-shell ") the 

 nauplius stage, although more difficult to examine, 

 is quite as well marked. 



Of the other groups of the Malacostraca, the Syn- 

 carida and Leptostraca are hatched in nearly the 

 adult form, but the Stomatopoda have a long series 



