66 OUTLINES OF NATURAL HISTORY. 



coloured spots. Immediately above the head is a folded 

 tube, which the animal can thrust out to a considerable 

 length. This tube (s) acts as a pipe or " siphon," by 

 which fresh water is carried to the gills. At the hinder 

 end of the foot we observe an oblong horny plate (o) with 

 numerous concentric lines upon it. This plate is known 

 as the "operculum" (Latin for a lid), and the function 

 which it discharges is obvious. When the animal, namely, 

 retires into its shell, the portion of the foot which carries 

 this horny plate is the last to be drawn in ; and as it fits 

 accurately into the mouth of the shell, the Wlielk is thus 

 protected by the operculum against injury. 



Over its back the Whelk carries a shell, which is com- 

 posed of a single piece, and is therefore said to be 

 " univalve." The shell has the general form of a cone, 

 the broadest end of which is turned toward the head of 

 the animal, whilst the pointed end is directed backwards. 

 In reality, the shell (fig. 27, B) is composed of a conical 

 tube, which is twisted in an oblique manner round a 

 central pillar, and which therefore forms a " spiral." The 

 first few turns of the shell are comparatively small, and 

 they constitute what is termed the "spire" (fig. 27, B, 

 «). The last turn of the shell is by far the largest (fig. 

 27, B, h), and as it contains the greater portion of the 

 body of the animal, it is termed the " body- whorl." All 

 the turns of the shell are in contact with one another, 

 and the body-whorl opens in front by a large oval aper- 

 ture, from which the animal can protrude itself, and which 

 is known as the " mouth " of the shell. The inner mar- 

 gin of the mouth is formed by the pillar round which the 

 whole sheU is coiled, and to this pillar the animal is 

 firmly attached by means of a special muscle. Lastly, 

 the mouth of the shell exhibits in front a well-marked 

 notch (fig. 27, B, n), which is for the passage of the 

 bi'eathing-tube or "siphon " already spoken of. 



Externally, the shell exhibits numerous lines or striae 

 running parallel with one another and with the spiral 

 turns of the shell. There is also a series of undulations or 

 folds which run in the direction of the length of the shell, 



