362 MOLLUSC A. 



the more specialized, e.g. Pulmonata (except Auriculidae, Siphonaria, Gadinia} 

 and Cavoliniidae. It may be present or absent in the same genus (Stomatella, 

 Vermetus, Conus). and may even be caducous in adults (Limacina helicina). 



The edges of the mantle are often folded over the shell, so as to 

 cover a part of it (many Fissurellidae, Marsenina, many Cypraeidae 

 and Marginellidae, Aplysia, and some Bullidae, various Pulmonates), 

 or the whole of it. In the latter case the edges fuse, and the shell 

 is enclosed in a sac. Such an internal shell is found in Cryptochiton, 

 Pupilia, most Lamellariidae, Pustule/via, Notarchus, Doridium, Gas- 

 tropteron, Philine, Pleurobranchus, and some species of Limax, and is 

 very generally much reduced in size, and quite incapable of receiving 

 the head and foot in retraction. Finally the shell and its sac may be 

 absent, and the visceral sac become secondarily symmetrical (Titis- 

 caniidae, Pterotrachea, Runcina, Gymnosomatous Pteropods and 

 Cymbuliidae, Pleurobranchaea, Nudibranchiata, Philomycidae, On- 

 chidiidae, Vaginulidae). In such cases the shell exists only in the 

 larva (in some Pulmonates it is never formed at all), and disappears 

 at the end of larval life. In most cases in which there is no shell 

 the pallial chamber (and groove) and ctenidium are reduced or absent; 

 e.g. in Nudibranchs where the ctenidium is said to be replaced by the 

 papillae of the mantle (i.e. of the dorsal integument) called cerata 

 (Fig. 328). In certain cases in which the larval shell is shed, 

 another persistent and internal shell is formed (Lamellaria, the first 

 spiny shell of which is called Echinospira). The attachment to the 

 shell is effected by the columellar or spindle muscle (Fig. 296), which 

 arises from the foot and is inserted into the columella in the spiral 

 forms, or when there is no columella to the internal surface of the 

 shell in a horse-shoe-shaped line (Patella), or over an oval area 

 (HaUotis, Fig. 296 S). 



The central nervous system consists typically of the three pairs 

 of ganglia in the head (Fig. 282) the cerebral supplying the 

 head and sense-organs, the pedal supplying the foot, and the 

 pleural which innervate the mantle and spindle muscle ; and of 

 two commissures which are completed ventrally to the gut and 

 contain ganglia in their course; these are the anterior or stomato- 

 gastric commissure, on which are developed the buccal ganglia, and 

 the posterior or visceral, on which may be developed the supra- 

 and sub -intestinal ganglia and one or more abdominal ganglia. 

 The stomatogastric commissure is connected with the cerebral 

 ganglia, and supplies the buccal mass and alimentary canal, while 

 the visceral commissure is connected with the pleural ganglia, and 



