200 INVERTEBRATE ANIMALS. 
breathing forms the young is protected by a small nautilus- 
shaped shell, within which it can entirely withdraw. It is 
enabled to swim about freely by means of two ciliated lobes 
springing from the sides of the head, and in this stage it is 
very like the permanent adult condition of the Ptevopoda (fig. 
108). 
As regards the shell of the Gasteropoda, the following points 
may be noticed. The shell is composed either of a single piece 
(univalve), or of a number of plates placed one behind the 
other (multivalve). 
The univalve shell is to be looked upon as essentially a hollow 
Fig. 104.—Gasteropoda, a@ Shell of the Turrite la communis, showing a round 
mouth ; 4 Shell of the common whelk (Buccinum undatum), showing the mouth 
notched for a respiratory siphon. 
cone, the apex of which is placed a little on one side. In the 
simplest forms, as in the Limpets, the conical shell is retained 
throughout life without any alteration. In the great majority 
of cases, however, the cone is considerably elongated, so as to 
form a tube, which may retain this shape (as in the “tooth- 
shell”), but which is usually coiled up into a spiral. The 
“spiral univalve” may, in fact, be regarded as the typical form 
of the shell in the Gasteropoda (fig. 104). The coils of the 
spiral are termed the “whorls,” and are usually more or less 
amalgamated on one side. In most cases, too, the whorls are 
