CHAP. IX 



POSITION OF THE SHELL 



245 



a bivalve shell. In Pliolas^ and in some species of Thracia^ 

 there is a small accessory hinge plate ; in the Polyplacophora, 

 or Chitons, the shell consists of eight plates (see Fig. 2, p. 8), 

 usually overlapping. A certain proportion of the Mollusca 

 have no shell at all. In many of these cases the shell has been 

 present in the larva, but is lost in. the adult. 

 The shell may be 



(1) External^ as in the great majority of both univalves and 

 bivalves. 



(2) Partly external^ idartly internal ; e.g. Homalonyx^ Hem- 



FiG. 148. — Aplustrum aplustre 

 L. Mauritius, showing the 

 partly internal sliell (S) ; F, 

 foot; LL, cephalic lappets; 

 TT, douhle set of tentacles. 

 (After Quoy and Gaimard.) 



Fig. 149. — Sigaretus laeviga- 

 tus Lam., showing- shell 

 partially immersed in the 

 foot ; F, anterior i^rolonga- 

 tion of the foot. (After 

 Souleyet.) 



pJiillia, some of the Natieidae^ Scutum^ Acera^ Aplustrum (Figs. 

 148 and 149). 



(3) Liternal ; e.g. Philine^ Gastropteron, Pleurohranclius^ 

 Aplysia^ Limax^ Arion^ Hyalimax^ Parynacella^ Lamellaria^ 

 Oryptochiton^ and, among bivalves, Chlamydocoyicha. 



(4) Absent ; e.g. all Nudihranchiata and Aplacophora^ many 

 Cephalopoda., a few land Mollusca, e.g. all Onchidiidae^ Phllomycus, 

 and Vaginula. 



The Univalve Shell. — In univalve Mollusca the normal form 

 of the shell is an elongated cone twisted into a spiral form round 

 an axis, the spiral ascending to the left. Probably the original 

 form of the shell was a simple cone, which covered the vital 

 parts like a tent. As these parts tended to increase in size, their 



