310 THE NATURAL HISTORY 



(or their analogues), and other portions of the animal 

 frame ; in the animals under consideration, this plan 

 is considerably modified, and instead of one cerebral 

 mass governing the whole system, we find half a dozen 

 ganglionic centres from which the nervous threads 

 radiate to the organs of sight, smell, hearing, and 

 touch, which appear to be well developed, and the 

 important systems of digestion, circulation, respiration, 

 reproduction and locomotion. When we have added 

 to this faint outline of the internal structure of snails 

 and slugs, that these animals are essential agents in 

 checking the redundancy of vegetation, removing de- 

 composing matter, both animal and vegetable, arrd 

 supplying dainty food to many other members of the 

 animal kingdom, we have perhaps said enough to 

 shew that they possess a high degree of interest. 



If we next make a passing allusion to the Pearl- 

 bearing Mussels, or Unios, it is chiefly with the 

 object of expressing our regret that they have not a 

 single representative in our Harting Fauna that we 

 are aware of. Many rivers and streams in the British 

 Islands, however, are, or have been, rich in them ; and 

 nearly two thousand years ago, Britain was celebrated 

 far and wide for its pearls, so much so, indeed, that 

 according to trustworthy historians of that remote 

 period, we are indebted to these precious objects for 

 the first hostile visits of the Romans to our shores, on 

 which occasions in the words of a very humorous 

 modern writer Caesar "broke in upon the natives 

 with considerable energy." Several rivers in the 

 British Islands are noticed in works on the subject, as 

 having been at one time the seats of pearl fisheries. 

 We gratefully record the fact that we not long since 

 received a valuable present of Scotch pearls from a 

 gentleman, who at the same time sent us the following 

 extract from the Cornhill Magazine (No. 80, August, 

 1 866) : " One pearl is on the average found in every 

 thirty shells, but as only one pearl in every ten is sale- 



