198 A GENERAL, VIEW OP THE 



Nature, we cannot overlook tlie animal which is tlieir sole archi- 

 tect. The shells of the more abundant species of Conchifera some- 

 times form the entire line of beach along the sea shore for miles, 

 and in many places they are burnt for lime. The Teredo navalis 

 bores by means of its shells into the hardest timber, and the genus 

 Fholas buries itself in the hard rock. IMany shells of Conchifera 

 are here before you, and they display great beauty in their forms 

 and colour. The recent, waved, and inner layers of the shell are 

 generally more pellucid, have greater lustre, and are more transpa- 

 rent than the exterior layers, which have been longer acted on by 

 the water and by the sand and gravel which wash over them. 

 Upon removing the outer layers, we often obtain the remaining 

 inner portions of a beautiful pearly lustre, constituting in fact mo- 

 ther-of-pearl. 



Pearls are formed by conchiferous animals. A little particle of 

 sand, perhaps, gains access between the mantle of the animal and 

 the last formed layer of the shell, the irritation it creates causes the 

 nacrious matter to be thrown out, and successive layers being depo- 

 sited a pearl is formed. The Chinese have a practice of forcing the 

 Swan Muscle to make pearls by throwing into its shell, when open, 

 three or four very small mother-of-pearl beads : in the course of a 

 year or two they are found covered with a crust perfectly resem- 

 bling pearls. 



Animals inhabiting univalve shells are termed Gasteropoda, be- 

 cause, like the Common Snail, for example, they creep upon a mus- 

 cular disc which extends along the lower surface of the body. Gas- 

 teropodous shells are, many of them, very splendid and exceedingly 

 varied in form and size. Immediately beneath the shell is the man- 

 tle of the animal by which it is secreted, and when the shell is 

 broken the mantle has the power of repairing it. 



Closely allied, in many respects, to the three last tribes are the 

 rieropoda, small delicate animals, swimming together near the 

 calm surface of tropical seas by means of curious wing-shaped mem- 

 branes, from which they derive their scientific appellation. 



The Cephalopoda approach nearest to fishes. The name is deriv- 

 ed from the circumstance of the feet of the animal closely surround- 

 ing the head. Several of the Cephalopoda inhabit a pecular kind 

 of shell, composed of a series of chambers, and constituting a multi- 

 locular or polythalamous shell ; for instance, the Argonaut and the 

 Nautilus. The Sepia officinalis is a very remarkable cephalopod 

 animal. The family is an extensive one, and will prove highly 

 interesting to those who have opportunities of studying it. Among 



