MOLLUSCA : GASTEROPODA. 289 



geal or cerebral, the infra-cesophageal or pedal, and the parieto- 

 splanchnic ; but there is a tendency to the aggregation of these 

 in the neighbourhood of the head. The organs of sense are 

 the two eyes, and auditory capsules placed at the bases of the 

 tentacles, the latter being tactile organs. 



The sexes are mostly distinct, but in some they are united 

 in the same individual. The young, when first hatched, are 

 always provided with an embryonic shell, which in the adult 

 may become concealed in a fold of the mantle, or may be 

 entirely lost. In the branchiate Gasteropods the embryo is 

 protected by a small nautiloid shell, within which it can en- 

 tirely retract itself; and it is enabled to swim freely by means 

 of two ciliated lobes arising from the sides of the head ; thus, 

 in many respects, resembling the permanent adult condition of 

 the Pteropoda. 



Shell of the Gasteropoda, The shell of the Gasteropods is 

 composed either of a single piece (univalve), or of a number 

 of plates succeeding one another from before backwards (mul- 

 tivalve). The univalve shell is to be regarded as essentially 

 a cone, the apex of which is more or less oblique. In the 

 simplest form of the shell the conical shape is retained without 

 any alteration, as is seen in the common Limpet (Patella}. 

 In the great majority of cases, however, the cone is consider- 

 ably elongated, so as to form a tube, which may retain this 

 shape (as in Dentalium\ but is usually coiled up into a spiral. 

 The "spiral univalve" (fig. 103) may, in fact, be looked upon 

 as the typical form of the shell in the Gasteropoda. In some 

 cases the coils of the shell termed technically the " whorls " 

 are hardly in contact with one another (as in Vermetus). 

 More commonly the whorls are in contact, and are so amalga- 

 mated that the inner side of each convolution is formed by the 

 pre-existing whorl. In some cases the whorls of the shell 

 are coiled round a central axis in the same plane, when the 

 shell is said to be "discoidal" (as in the common fresh- 

 water shell Planorbis). In most cases, however, the whorls 

 are wound round an axis in an oblique manner, a true spiral 

 being formed, and the shell becoming " turreted," " trochoid," 

 " turbinated," &c. This last form is the one which may be 

 looked upon as most characteristic of the Gasteropods, the shell 

 being composed of a number of whorls passing obliquely round 

 a central axis or " columella," having the embryonic shell or 

 "nucleus" at its apex, and having the mouth or "aperture" 

 of the shell placed at the extremity of the last and largest of 

 the whorls, termed the " body-whorl." The lines or grooves 

 formed by the junction of the whorls are termed the " sutures," 



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