CRABS AND SHRIMPS. 171 



CHAPTER XL 



CRABS AND SHRIMPS. 



It is always interesting to trace the varied forms and 

 •conditions under which any particular function is per- 

 formed ; and particularly to mark, in 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 Mollusca and Insects; 

 and will now examine some more, as they appear in the 

 Crustacea. 



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

 Prawn, you will see that it is furnished, like that of 

 Insects, with jointed antenna? ; 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 antenna? ter- 

 minate each in three many-jointed bristles, in structure 

 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 internal 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 great Eatable Crab 

 each pair is very small, and they are dissimilar. 



Now, taking the last-named animal as the representa- 

 tive of his class, let us examine one of his inner antennae 

 first. It consists of a jointed stem and a terminating 

 bristle ; the latter furnished with small hairs common to 



