84 



COLUMELLA. UMBILICUS. 



2. The arrangement of these shells varies much ; sometimes 

 they are composed of many pieces, at others, of a single piece , 

 sometimes they are symmetrical, at others not symmetrical, and 

 in this last case, in proportion as they are elongated, they are 

 rolled or twisted upon themselves, and constitute a spirally 

 twisted cone. This last form, which is 

 seen in the shell of the snail, is the most 

 common in gasteropods, and presents 

 numerous varieties. Sometimes the part 

 which grows less rapidly and upon which 

 the cone rolls itself, is full (Jig. 19), and 

 sometimes empty ; it is called columella 

 or pillar (c), and, when empty, we give 

 d the name of umbilicus to this opening 

 (fg. 20, ti). If the turns of the shell 

 remain in the same plane, the spire is 

 flat or concave, and the general form of the 

 shell is discoid, resembling a disk, as in the 

 planorbis (Jig. 29) ; sometimes the height 

 of each turn completely envelopes thepreceding involutions, so that 

 the spire is concealed ; but in most cases the turns are inclined 

 towards the edge of the columella, and there results an oblique 

 spire, which is more acute in proportion as 

 the cone is less rapidly widened (Jig. 19). 

 Those shells are called turbinated, in 

 which the first turns are raised up above 

 the succeeding ones (Jig. 19), forming a 

 projecting spire, and it is remarked that 

 the turns or whorls are almost always di- 

 rected from the right side. Sometimes how- 

 ever the spire of these turbinated shells is 

 turned from the left side of the animal 

 (Jig. 25), and in this case they are said to 

 be perverse. 



Fig. 19. SECTION OF 



A TURBINATED SHELL. 



Fig. 20. NATICA. 



Explanation of Fig. 19. Section of a turbinated shell to show how it is 

 spirally wound upon itself': b. the mouth or aperture of the shell ; c. the 

 columella ; d. l^e part of the columella included in the last turn of the spire 

 s. turn of the spiie next to the last; p. the apex or summit of the shell; 

 i. part of tne columella. 



2. Do the shells of ga'sterqpods consist of a single piece ? What is meant 

 by the colume'tla ? What is the umbili'cus ? What is meant by a tu binated 

 hcl! ? What is meant by a perverse shell ? 



