753 



CHAPTER XXXVII. 



Class II. Gastropoda (or Gasteropoda). 



The members of this class are Mollusca with a more or less distinct 

 head, and a generally unsymmetrical body. The mantle is never 

 divided into two lobes, and the shell, when present, is univalve. The 

 "foot" is well developed, and usually has the form of a broad hori- 

 zontally-flattened ventral disc, upon which the animal creeps. 



The body in the Gastropods is composed of three principal por- 

 tions — a head, foot, and visceral sac — the last of these being more 

 or less completely protected by a fold of the dorsal integument, con- 

 stituting the " mantle." The body is distinctly unsymmetrical, and 

 the mantle is never divided into a right and left lobe, while the vis- 

 ceral sac is often coiled up spirally. The foot is typically in the 

 form of a broad flattened muscular disc, developed upon the ventral 

 surface of the body, and not exhibiting any distinct division into 

 parts. In the Heteropoda, however, and in the Wing-shells (Strom- 

 bides), the foot exhibits a division into three portions : an anterior, 

 the " propodium " ; a middle, the " mesopodium " ; and a posterior 

 lobe, or " metapodium." In the Heteropoda, the foot is flattened, 

 and forms a ventral fin, by means of which the animal swims, back- 

 downwards. 



In some, again, the upper and lateral surfaces of the foot are 

 expanded into muscular side-lobes, which are called "epipodia." 

 In the Pteropodous division of the Gastropods the epipodia are the 

 only portions of the foot which are developed, and they constitute a 

 pair of fins attached to the sides of the head. In many cases the 

 metapodium, or posterior portion of the foot, secretes a calcareous, 

 horny, or fibrous plate, which is called the "operculum" (fig. 639, 

 o), and which serves to close the orifice of the shell when the animal 

 is retracted within it. 



The shell of the Gastropods is a secretion from the mantle, and 

 is always present in the embryo. It is, however, wanting in the 

 adults of the Nudibranchs and in some other forms, and it is in other 



vol. 1. 3 p, 



