INTRODUCTION 



TO 



SHELL-BEARING BRITISH MOLLUSCA. 



A Common garden Snail, observed crawling and feeding, is easily seen 

 to possess a distinct head, upon which are four feelers or horns, with 

 eyes at the end, and underneath which is a mouth ; a broad disc, ex- 

 tending the whole length of thfe body, is the organ of locomotion ; a 

 spiral or coiled shell surmounts the whole. 



If, on the other hand, an Oyster be opened, no distinct head, or place 

 for eyes or mouth, can be seen, but only a seeming mass of pulp, with 

 a round gristle in the centre (the muscle of attachment), the former 

 enclosed in a soft skin with a double fringe round the edge (mantle), 

 placed within a shell composed of two pieces. 



All Mollusca are more or less completely represented by these two. 

 The Snail and its class are named Gasteropoda (abdomen-creeping), and 

 their shells Univalves; the Oyster and its class, Acephala (headless), 

 and their shells Bivalves. We begin with the latter class at — 



Plate I. 



Pholadidce, or "Borers/ 5 1-13, have the power of piercing and in- 

 habiting holes in submarine wood, rocks, and stones. The hinder part 

 of the body is lengthened out in the form of a double tube, with open- 

 ings for the ingress and egress of fluids. Teredo, 1-6, has short valves, 

 two pallets fixed at the sides of the siphons, and lines its hole with a 

 shelly tube. Pholas and Pholadidea, 8-13, have long valves and some- 



