CRABS AND SHRIMPS 219 



six rectangular teeth, and each tooth bears on its hinder 

 face a long spine articulated to it, and most delicately 

 plumose all along its sides. The hindermost pair of spines 

 are short, and are set close together, side by side. Be- 

 sides these jointed spines, each lateral angle of the caudal 

 lozenge-shaped tail-plate projects into a spine-like tooth. 



Though the individuals before us are all in the same 

 state as to the stage of their development, there is some 

 difference in size, indicating, doubtless, a corresponding 

 diversity in age. "We will isolate a few of the largest, 

 and put them into a glass trough for microscopical exam- 

 ination. 



The largest, during the few minutes which I have occu- 

 pied in the process of dipping them out, has undergone a 

 metamorphosis. You observed that, after skipping about 

 the trough for a few moments, it sank quietly to the bot- 

 tom, where it lay on its back; the next thing that you 

 see is a much more crab-like animal, more opaque, redder, 

 much larger, but lying on its back in the very spot where 

 a moment before you had seen a Zoea; while close by it 

 lies the transparent filmy skin which has been cast off. 



The new animal is evidently now in its final state, need- 

 ing only development of its parts, which it would obtain, 

 if in freedom, by successive moults, to acquire the adult 

 form. 



If we now submit the exuviae in detail to a power of 

 220 diameters, we shall obtain some interesting views of the 

 structure. The slough of the eyes in particular presents 

 one of the most exquisite objects that you can behold. 

 They are somewhat pear-shaped, with the facetted por- 

 tion well defined. It is the appearance of these facets, 

 varying according as the perfectly hexagonal outline of 



