164 . LIFE BY THE SEASHORE. 



do not usually occur in shoals, and are often littoral. They 

 show much greater diversity of structure and habit than 

 the prawns, and have apparently been subjected to a much 

 keener process of selection. There are in consequence few 

 groups of marine animals which illustrate the problems of 

 evolution more clearly, or afford more fascinating objects 

 for study. One may read many books on the Doctrine of 

 Descent, and yet remain untouched by the charm of the 

 theory, but few persons can, I imagine, toil over the 

 structure and affinities of these Crustacea without suddenly 

 becoming conscious of the grandeur of the generalisation, 

 of its power of unifying what previously seemed insignificant 

 details. 



We shall now proceed to consider successively typical 

 British representatives of the Decapoda. 



The members of the sub-order Natantia all fall into the 

 family Caridida3 which has the characters of the sub-order. 



A large number of genera are included in this family, 

 but it is only possible for us to consider a few of them. 

 We may repeat, however, that the great interest of these 

 forms is that on the one hand they show close relationship 

 to the next lower order of Crustacea, the Schizopoda, and 

 on the other they markedly resemble the Eeptant Decapods. 

 This is especially true of the lowest forms, notably the 

 curious shrimp Peneus, which seems to stand half-way 

 between the Schizopods and the crayfish and lobster. This 

 shrimp is, however, a Mediterranean form, and only occurs 

 very rarely in the South-west of Britain. 



It is worth while to notice here that there are a number 

 of interesting Crustacea which are rare in Britain, and are 

 confined to the South and West. Such forms are almost 

 always Mediterranean species, and we may say generally 

 that our littoral fauna is of two types, the Mediterranean 

 type, which predominates on the South and West, and the 

 Northern, or Scandinavian type, which predominates on the 

 North and East. In addition, on the West we find certain 

 peculiar animals which are not truly members of our fauna, 

 but are brought, more or less passively, by the Gulf Stream. 

 Animals which occur all round our coasts may generally be 

 assumed to be common to the Scandinavian and Mediter- 

 ranean faunas, while our East Coast rarities are Scandinavian 



