POSITION OF THE SHELL 



245 



shell. In Pholas, .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 Molhisca 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 lie 



(1) External, as in the great majority of Ijoth univalves and 

 bivalves. 



(2) Fartly external, iwrthj internal; e.g. Honudonyx, Hem- 



FiG. 148. — Aplustnan nplustre 

 L., Mauritius, showing the 

 partly internal sliell (S) ; F, 

 foot ; LL, cephalic lappets ; 

 TT, double set of tentacles. 

 (After Quoy and Gainiard. ) 



Fig. 149. — Siijaretus laeviga- 

 tus Lam., showing shell 

 partially immersed iu the 

 foot ; F, anterior pro- 

 longation of the foot. 

 (After Souleyet.) 



phillia, some of the Naticiclae, Scutum, Accra, Aplustrum 

 (Figs. 148 and 149). 



(3) Internal ; e.g. Fhiline, Gastropteron, FleurohrctncJms, 

 Aplysia, Liniax, Arion, Hyalimax, Parmacella, Lamellaria, 

 CryptocJiiton, and, among bivalves, Chlamydoconclia. 



(4) Absent; e.ij. all Nudihranchiata and AjjlacopJiora, many 

 Cephcdopoda, a few land Mollusca, e.g. all Onchidiidae, Philomycus, 

 and Varjinula. 



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, tlieir 



