GASTEROPODS 



331 



Buccinum undatum : si, siphon. 



the animal draws in water to supply the gills, lying just back 

 of the siphon and concealed under the mantle in the cavity 

 already spoken of as the mantle cavity. With the exception of 

 that portion of the mantle which constitutes the siphon, no part 

 of its margin, it will be observed, can be seen, the edge of the 

 shell completely hiding it from view. 



In some genera the siphon is exceedingly long in fact, longer 

 than the body of the animal. In these cases the mollusk is 

 generally fond of burying 

 itself in the sand and main- 

 taining communication with 

 the world above only by 

 means of this long, fleshy 

 tube. In some genera the 

 siphon is protected by an 

 elongation of the shell, as in 

 Fasdolaria (Plate LXXVI). 

 An extreme example of this is given in the figure on page 343, 

 which represents a species of Murex, where the siphon is not only 

 protected by the shell, but the shell itself is further protected by 

 the double row of sharp spines with which it is beset. 



There are some genera which possess no siphons at all, in all 

 of which cases there is no notch in the shell opening, the aperture 

 being round, as in Natica ami in Littorina (pages 368, 371). The 

 mollusks of this type make a fold in the forward part of the 

 mantle which serves as a siphon. Such a muscular folding in 

 the mantle would be technically called a functional siphon. 



On the posterior dorsal side of the foot is attached a horny 

 plate, of concentric structure about a central or subcentral 

 point or nucleus. This is called the operculum, and serves 

 to close the entrance of the shell when the animal is withdrawn, 

 the object no doubt being protective. The opercula vary to a 

 great extent in the different genera ; in some cases they are cal- 

 careous, in others cartilaginous. When the aperture of the shell 

 is very large it often happens that the operculum does not entirely 

 close the opening, as in Fiilgur, but in Buccinum it is a close fit. 

 The operculum is entirely wanting in the Pulmonata, one of the 



