MOLLUSCA PROPER. 201 



hatched before they leave the parent, and, when lir.;t liberated, ai-e 

 ciliated and free-swimming. 



The habits of the Lamellihranchiata are very various. Some, such as 

 the Scallops (J'ccten), habitually lie on one side, the lower valve being 

 usually the deepest, and the foot rudimentary or wanting. Others 

 are fixed to the bottom of the sea by the substance of one of the 

 valves. Others, such as the common Mussel, are moored to some 

 foreign object l:iy a tuft of silky fibres, constituting a "byssus." 

 Many, such as the Gapers (J/y«) and Razor-shells (So/en), spend 

 their existence sunk in the sand of the sea-shore or the nuid of 

 estuaries. Others, such as the Pholades, bore holes in rfick or 

 wood, in which they live. Finally, many are permanently free and 

 locomotive. 



Class II. Gasteropoda. 



This class includes an enormous number of Molluscs, such as the 

 I.aiid-snails, Sea-snails, Whelks, Limpets, Slugs, Sea-lemons, &c., 

 which agree in many fundamental characters, but nevertheless pre- 

 sent many striking differences. From the very common occurrence 

 of a shell composed of a single piece, the Gasteropoda are often spoken 

 of in a general way as the " Univalve " Molluscs. In many, Ikjw- 

 ever, there is either no shell at all, or one so small that it would 

 not generally be recognised as such ; and in a few {Chiton) the shell 

 is composed of eight pieces (" multivalve "). In ncjne, liowever, is 

 the shell composed of two pieces or " bivalve." The large majority 

 (jf the Gasteropoda are further distinguished by the great develop- 

 ment of the foot, which constitutes a broad flattened disc u])on 

 which they creep about, as may readily be observed in the common 

 Slugs or Snails (fig. 142). Some, however, have the foot nuich 

 modified and adapted for swimming. In many cases, also, the foot 

 carries behind a horny or shelly plate which is known as the " ojier- 

 culum" (fig. 140, o), and which serves to close the shell when the 

 animal is withdrawn within it. 



The head in most of the Gasteropoda, unlike the Bivalves, is very 

 distinctly marked out, and carries two long feelers, and two ej'es, 

 often placed upon stalks (fig. 142). The month, also, differs from 

 that of the Bivalves in Vicing furnished with a singular ajiparatus 

 of teeth, constituting what is known as the " odoutn|ili()re " (fig. 

 141), or " lingual ribbon. This consists essentially of a number of 

 chitiuous teeth, of different shapes in different species, supported 

 upon a kind of strap which can be made to work backwards and 

 forwards over a cartilaginous cushion, thus acting like a chain-saw. 

 In addition to the odontophore there are sometimes liorny jaws as 

 well. The mouth (see fig. 129) leads by a gullet into a distinct 



