Ixii INTRODUCTION. L^H. 



and Bythinia are similarly infested; but they are of 

 various sizes, and some of them have different habitats. 

 In the sea, a small kind of pea-crab {Pinnoteres vete- 

 rum) is frequently found inside the mantle of Cyprina 

 Islandica, Modiola modiolus (or the great horse-mussel), 

 and Pinna pectinata, taking up its abode in these snug 

 quarters for parasitic purposes, and not (as was imagined 

 by the too credulous Pliny) in order to warn the Pinna of 

 the approach of its foes, like a faithful friend or watch- 

 dog. Professor KoUiker has lately noticed in the shells 

 of several kinds of Mollusca, both univalve and bivalve, 

 certain vegetable parasites, which he regards as unicellular 

 fungi. They form minute tubes, which run straight 

 through the pores or fibres of the shell. He thought 

 it probable that these vegetable parasites dissolve the car- 

 bonate of lime contained in the shell by means of an acid 

 which they may have the power of secreting. But the pro- 

 cess by which shells are perforated by vegetable as well 

 as animal organisms does not seem to be understood, and 

 requires much elucidation in a chemical point of view. 



Uses to Man. — {Food.) — We naturally consider our- 

 selves (as the ^' lords of the creation'^) the sole pivot round 

 which all other creatures turn, without much sympathy 

 with them or regard for their wants and feelings. But 

 the countless and complicated links of the chainwork in 

 which all Nature is involved are so closely and wonder- 

 fully connected together, that not one of them can be 

 broken or displaced without interfering with the eco- 

 nomy of the whole. Much of the animal food which we 

 consume has been nourished at the expense of other 

 creatures, which in their turn have subsisted upon smaller 

 organisms ; and this process of destruction is repeated 

 until the bottom of the scale of animal life has been 

 reached. Then the varied and inexhaustible stores of 



