CEABS AND SHEIMPS. 197 



CHAPTER XI. 



CEABS AND SHEIMPS. 



It is always interesting to trace the vai'ied fonns 

 and conditions under which any particular function is 

 performed ; and particularly to mark, ia creatures very 

 remote from us in the scale of being, the organs devoted 

 to the senses which are so requisite to our own comfort. 

 We have already seen some of these diversities, in ex- 

 amples taken from the classes MoUusca and Insects / 

 and will now examine some more, as they appear ia 

 the Crustacea. 



If you look at the head of a Crab, a Lobster, or a 

 Prawn, you will see that it is furnished with jointed 

 antennae, like that of* Insects ; but whereas in insects 

 there is never more than a single pair, in the creatures 

 of which I am speaking there are two pairs. In the 

 Prawn you may suppose, at first sight, that there are 

 four pairs ; but that is because the internal antennse 

 terminate each in three many-jointed bristles, in struc- 

 ture and appearance exactly like the bristles of the 

 outer pair, two of the three being nearly as long as the 

 outer, while the third is short. In the Lobster, the in- 

 ternal are two-bristled, both bristles rather short, while 

 the external are very long. In the Flat-crabs each pair 

 is simple, the inner minute, the outer long. In the 



