=Lacaze-Duthiers=, Otocystes ou Capsules auditives des Mollusques (Gastéropodes): Arch. Zool. Exp. Gén. i. 1872, pp. 97–166. „ „ Du système nerveux des Mollusques gastéropodes pulmonés aquatiques: _ibid._ pp. 437–500. =P. Pelseneer=, Recherches sur le système nerveux des Ptéropodes: Arch. Biol. vii. 1887, pp. 93–130. „ Sur la valeur morphologique des bras et la composition du système nerveux central des Cephalopodes: Arch. Biol. viii. 1888, pp. 723–756. =H. Simroth=, Ueber die Sinneswerkzeuge unserer einheimische Weichthiere: Zeit. wiss. Zool. xxvi. 1876, pp. 227–348. =J. W. Spengel=, Die Geruchsorgane und das Nervensystem der Mollusken: Zeit. wiss. Zool. xxxv. 1881, pp. 333–383. CHAPTER VIII THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS The digestive tract, or, as it is often termed, the alimentary canal or gut, is a very important feature of the Mollusca. It may be regarded as consisting of the following parts: (1) a _mouth_ or _oral aperture_: (2) a _throat_ or _pharynx_; (3) an _oesophagus_, leading into (4) a _stomach_, (5) an _intestine_ and _rectum_, ending in (6) an _anus_. The primitive positions of mouth and anus were presumably at the anterior and posterior ends of the animal, as in the Amphineura and symmetrical Mollusca generally. But the modifications of original molluscan symmetry, which have already been referred to (p. 154, compare pp. 245, 246), have resulted in the anus becoming, in the great majority of Gasteropoda, twisted forward, and occupying a position on some point in the right side in dextral, and in the left in sinistral species. The process of digestion, as the food passes from one end of the tract to the other, is performed by the aid of the secretions of various glands, which open into the alimentary canal at different points in its course. The principal of these are the _salivary glands_, situated on the pharynx and oesophagus, and the _liver_, _biliary_ or _hepatic gland_, connecting with the stomach. With these may be considered the _anal_ and _ink-glands_, which, in certain genera, connect with the terminal portion of the rectum.