663 MALACOLOGY. 



In some cases, the shell is internal, or lodged in the skin, but, 

 generally, it is external, and affords complete protection to the 

 animal. 



Mollusks which, like the Cuttle-fish, (see Chart,) have no 

 outer shell, are said to be naked; those having a shell, are 

 called Testaceous or Conchiferous. The shell varies in form, 

 the shape being determined by the animal itself. Sometimes 

 it resembles a shield that covers the back of the Mollusk, 

 but more frequently it is like a conical tube spirally twisted; 

 or it may be composed of two distinct pieces united by a joint ; 

 hence, the arrangement of these animals into Univalves and 

 Bivalves. The first, or the MOLLUSCA CEPHALATA, have a dis- 

 tinct head, bearing lips or jaws, and are furnished with eyes 

 and tentacula; the Bivalves, or the MOLLUSCA ACEPHALA, have 

 a more simple organization. These have no distinct head, 

 and are destitute of jaws, and other hard parts of a mouth. 

 The shells are often ornamented with colors variously dis- 

 posed, the animals themselves being furnished with the materials 

 for beautifying as well as constructing their outward cover- 

 ings. The skin is full of pores, containing colored fluids, which, 

 penetrating the calcareous substance before it hardens, form its 

 variegated tints. The regularity of the markings is admirable. 

 It is accounted for by the fact, that the pores containing the col- 

 ored matter are arranged in the skin of Mollusks with undeviating 

 order, as the spots upon the leopard, or the stripes upon the tiger. 

 When the liquid exudes, it stains the shell ; and the uniformity 

 of pattern in the shell results from the order in which the pores 

 are placed in the mantle. The numerous spines or digitations 

 found in many of the shells, (see Murex and Pteroceras, on the 

 Chart,) are formed by the prolongations of the mantle bearing 

 upon its edges the material for this calcareous deposit. 



The parts of a univalve shell, are (I) the body or lower part; 

 (2) the spire or tapering portion; (3) the turns or whorls-, 

 (when the lower whorls of the spire are pressed into the body 

 whorl or turn, they are said to be refuse;) (4) the suture or line 

 of junction of the turns; (5) the columellar or pillar, the axis of 

 the shell ; (6) the mouth or aperture with its peristone or margin, 

 which may be complete or not, and may be described as forming 

 an outer lip, and an inner lip; (7) the lid or operculum, (from 

 operior, to cover,) the plate or door with which some species 

 close the aperture. The spiral turns may be smooth, or vari- 

 ously marked with striae, laminae, ribs, nodosites, or spines, the 

 markings being longitudinal or transverse. In its natural posi- 

 tion, the mouth is beneath and forward, the spire painting back- 



