394 GASTEROPODA. 



CLASS GASTEROPODA. 



[Ety. gaster, under side of body ; pous, foot.] 



THE locomotive organ in the Gasteropoda consists of a broad, muscular under - 

 surface, or foot, upon which the animal creeps with a gliding motion. The expan- 

 sion and contraction of the muscles may be seen when a snail is moving over glass. 

 This form of the foot is the most important characteristic of the Gasteropoda. The 

 head is distinct, and usually furnished with tentacles and eyes. The mouth is on 

 the lower surface, and is often furnished with one or two teeth, or jaws, in the upper 

 part, and a ribbon-like tongue, with minute silicious teeth on its upper surface, which 

 are used with the upper teeth in separating the food. The teeth on the tongue are 

 called the lingual teeth. 



The body is generally much larger on one side than on the other, which produces 

 a spiral shell in the growth of the animal, because the shell is secreted at the edge 

 of the mantel. The shell nearly always consists of one piece (univalve), forming a 

 conical tube, twisted spirally ; but the tube is not perfect, because the inner wall of 

 each whorl is formed of the preceding whorl with only a thin coating of shelly 

 matter. Sometimes the tube is rolled in a plane, producing a discoid shell ; and 

 there are all grades of form, from the discoid to the upright. The right side of the 

 animal is usually the larger, and the shell produced is dextral ; but in some species 

 and in some abnormal specimens of dextral species, the spire is turned in the op- 

 posite direction, and the shell is therefore called sinistral. The winding of the tube 

 in the spiral shells as the animal grows, produces a central axis, which is called the 

 columella. It extends from the apex to the base, and forms the inner margin of 

 the aperture. The columella is usually hollow, and terminates at the base of the 

 shell with a small opening, called the umbilicus. The margins of the aperture are 

 called the lips. When the columella forms the inner lip, it is called the columellar 

 lip. The outer lip forms the convexity of the shell. Sometimes the lips are con- 

 tinuous, and sometimes the outer lip is more or less deeply notched ; and both lips 

 may be furnished with teeth or denticulated edges. The last whorl of the shell is 

 called the body whorl, from its receiving the body of the animal, and the remaining 

 whorls constitute the spire. The line which separates the whorls is the suture. 

 Many Gasteropoda have a calcareous plate attached to the hinder part of the foot, 

 which closes the aperture when the animal retracts itself within the shell ; this cover- 

 ing is called an operculum. 



The Gasteropoda are divided into two subclasses : the Heteropoda and Gaster- 

 opoda proper. The Heteropoda, also called the Nucleobranchiata, are all inhabitants 

 of the ocean, and usually have a shell covering only the essential organs of the body. 

 They swim rapidly near the surface of the water with the back downward, and 

 when the foot is present it is used to attach the animal to floating sea-weeds. 



The Gasteropoda proper are divided into two orders: one breathing air, the 

 Pulmonifera; and the other water, the Branchifera. The Pulmonifera include the 



