CRABS AND SHRIMPS. 171 



CHAPTER XI. 



4 

 CRABS AND SHRIMPS. 



It is always interesting to trace the varied forms and con- 

 ditions under which any particular function is performed ; 

 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 examples 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 In- 

 sects, with jointed antennas ; 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 antennae terminate 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 representative 

 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 the 



