CRABS AND SHRIMPS. 189 



Lends down. Its extremity gradually tapers into two 

 straight, sub-parallel, stiff spines, about as long as the 

 carapace itself, each terminating in a hooked point. 



The abdomen ends in a spinous plate, which is very 

 elegantly lozenge-shaped and beset with spines. Each of 

 the two latero-posterior edges of the lozenge is cut into 

 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 hindmost pair of spines 

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

 sides these jointed spines, each lateral angle of the 

 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 examination. 



The largest, during the few minutes which I have 

 occupied in the process of dipping them out, has undergone 

 a metamorphosis. You observed that, after skipping about 

 the trough for a few minutes, it sank quietly to the bottom, 

 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, 

 needing 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 portion 

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



