„ „ 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.
1. The mouth is generally, as in the common snail and periwinkle, placed on the lower part of the head, and may be either a mere aperture, circular or semicircular, in the head-mass, or, as is more usual, may be carried on a blunt snout (compare Fig. [6], p. 10, and Fig. [68], p. 159), which is capable of varying degrees of protrusion. From the retractile snout has doubtless been derived the long proboscis which is so prominent a feature of many genera (compare Figs. [1], B, and [99]), and in some (e.g. Mitra, Dolium) attains a length exceeding that of the whole body. As a rule, Mollusca provided with a proboscis are carnivorous, while those whose mouth is on the surface of the head are Vegetable feeders, but this rule is by no means invariable. The mouth is thickened round the aperture into ‘lips,’ which are often extensile, and appear capable of closing upon and grasping the food. In the Pelecypoda the mouth is furnished, on each side, with a pair of special external lobes, the ‘labial palps,’ which appear to be of a highly sensitive nature, and whose object it is to collect, and possibly to taste, the food before it passes into the mouth.