486 MULTILOCULAR -SHELLS. 



animal withdraws itself, the plate springs forward by the 

 elasticity of its pedicle, and closes the aperture. This 

 curious structure, and also the plaits of the mouth, which 

 are intimately connected with it, are not formed until the 

 animal has nearly reached maturity. It is best exhibited by 

 breaking off the outer part of the aperture to the distance 

 of about half a whorl, when it will generally be found free ; 

 but in order to exhibit it behind the columella in its natural 

 position, when the animal is exserted, it is necessary to kill 

 the animal in that situation, and then suffer it to dry before 

 the outer lip is broken off, and the pedicle will thus become 

 fastened to the side, by means of the dried mucus ; it may, 

 however, at any time be relaxed by a little moisture, when 

 it will instantly resume its elasticity, and spring from its 

 concealment."* 



IV. MULTILOCULAR SHELLS. 



When the cavity of a shell is divided into cells by trans- 

 verse septa or partitions, on a preordained plan, the shell is 

 said to be multilocular, polythalamous, or chambered. (Fig. 



102, b.) There is a multilocular 

 Fig. 102. shell amongst our native aquatic 



Gasteropods ; but, with this ex- 

 ception, chambered shells are the 

 produce of the Cephalopods. 



The shell is external or inter- 

 nal. In the latter case it is 

 white. 



The external shell is spiral, 

 and either discoid or revolute. 

 When revolute, the whorls are 

 twisted backwards into a spire 

 which is contained within the 

 outer whorl. Ex. Nautilus. 



The form of the septa is various, but the terms used to 

 express the variations explain themselves. The chambers 

 communicate by means of a siphon (Fig. 102 c.) which per- 

 forates the septa and runs through the chambers ; and accord- 

 ing to the position of the spot where the perforation is made, 

 it is named dorsal, central, ventral, or lateral. The recent 

 shells of this description are few in species, nor are the spe- 

 cies very numerous in individuals ; but the fossil kinds are 

 many and abundant, and some of them have been of a size so 



* Gray in Zool. Journ. i. 212 



