OF INVERTEBRATE ANIMALS. 407 



many persons when walking along the roads, on a fine summer's evening, after 

 the fall of a refreshing shower of rain, will have observed, amongst the numerous 

 species of Snails and Slugs that then appear slowly gliding over the moist foliage, 

 a very large Slug (Limosella), variegated like Tortoise-shell. In the back of 

 this Slug there is a shell, a very simple one, but it may be looked upon as the 

 commencing or vanishing point of shells amongst these animals. If we now go 

 to some lake or pond, and pluck a leaf of the Water-lily, it is a great chance but 

 we find upon it the fresh-water Limpet (Patella J. This animal will supply us 

 with a shell almost flat, but still a little concave. In some other forms of 

 Limpets we shall find the shell more concave, and presenting the form of a cone 

 with a very enlarged base. From this we may trace shells which gradually 

 present more acute cones, as in the Bonnet and Chambered Limpets {Patella 

 nereitoidea and P. Chinensis). A curve now begins to take place, and is well 

 seen in the Tooth-shell (Dentaliurri). In some of the Worm-shells (Serpulce) we 

 have the commencement of the spire. The spire may be next seen in a simple 

 form in the Sea-ear (Haliotis). It becomes more developed in the Nereides 

 (Nereidce). In the Snails and Periwinkles it is still more developed, and when 

 we have arrived at these forms we may yet trace its complication through the 

 Common Whelks (Buccinum) to the elegant forms of the Dippers (Bullce) and 

 Cowries (Cyprcea). 



The shells of the Univalve Mollusca are many of them very highly prized ; 

 there is one in a museum at Paris belonging to the Cone tribe, which is not more 

 than two inches long, and has been A'alued at three hundred guineas ; many 

 others exist, valued at from twenty to two hundred guineas. In some parts of 

 the world they are employed as a medium of circulation, instead of coin. The 

 Cowries are mostly used for this purpose. But. many of them serve for more 

 useful purposes. In France a large species of Snail (Helix pomatia) forms a 

 delicious article of diet ; whilst the Periwinkle and the Whelk are esteemed in 

 our own country. 



The Bivalves, of which we have many familiar examples, as in the Oyster, the 

 Cockle, the Muscle, &c, may be divided into two sections ; first, those which 

 bore or live in rocks, wood, mud, &c, and those which live free in water, and 

 are enabled to walk about by^ the aid of a foot, which projects from between 

 their shells. 



Among those which bore we find the Ship- woi*m (Teredo navalis). These 

 animals are supplied with two little shells, which, acting like rasps, enable it to 

 penetrate wood. They attack wood wherever it is found in the' ocean, and seem 

 to occupy it more as a resting place than for any other purpose. 



Another kind of these Bivalves attack even a harder material than wood, and 



3h2 



