2. The Pharynx, Jaws, and Radula.—Immediately behind the lips the mouth opens into the muscular throat, pharynx, or buccal mass. The pharynx of the Glossophora, i.e. of the Gasteropoda, Scaphopoda, and Cephalopoda, is distinguished from that of the Pelecypoda,[318] by the possession of two very characteristic organs for the rasping or trituration of food before it reaches the oesophagus and stomach. These are (a) the jaw or jaws, and (b) the radula,[319] odontophore, or lingual ribbon. The jaws bite the food, the radula tears it up small before it passes into the stomach to undergo digestion. The jaws are not set with teeth like our own; roughly speaking, the best idea of the relations of the molluscan jaw and radula may be obtained by imagining our own teeth removed from our jaws and set in parallel rows along a greatly prolonged tongue.[320]
In nearly all land Pulmonata the jaw is single, and is placed behind the upper lip. If a common Helix aspersa be observed crawling up the inside of a glass jar, or feeding on some succulent leaf, the position and action of the jaw can be readily discerned. It shows very black when the creature opens its mouth, and under its operation the edge of a lettuce leaf shows a regular series of little curved indentations, in shape not unlike the semicircular bites inflicted by a schoolboy upon his bread and butter. The jaw of Helix (Fig. [107], B) is arched in shape, and is strengthened by a number of projecting vertical ribs. That of Limax (A) is straighter, and is slightly striated, without vertical ribs. In Bulimulus (C) the arch of the jaw is very conspicuous, and the upper edges are always denticulated; in Orthalicus there is a central triangular plate with a number of overlapping plates on either side; in Succinea (E) there is a large square accessory plate above the jaw proper. The form of the jaw is peculiar not only to the genus but to the species as well. Thus the jaw of H. aspersa is specifically distinct from that of H. pomatia, and that of H. nemoralis is distinct from both. Wiegmann has observed[321] that in young Arion, Limax, and Helix, the jaw consists of two pieces, which coalesce by fusion in the adult, thus indicating a stage of development in advance of the double jaw which is found in most of the non-pulmonate Mollusca. In all fresh-water Pulmonata there are two small accessory side plates besides the jaw proper (Fig. [107], F).
Fig. 107.—Jaws of various Pulmonata: A, Limax (gagates Drap., Lancashire, × 15); B, Helix (acutissima Lam., Jamaica, × 15); C, Bulimulus (depictus Reeve, Venezuela, × 20); D, Achatina (fulica Fér., Mauritius, × 7); E, Succinea (elegans Riss., Aral District, × 30); F, Limnaea (stagnalis L., Cambridge, × 30).
Nearly all the non-carnivorous Prosobranchiata, land, fresh-water, and marine alike, are provided with two large lateral jaws. Many of these are sculptured with the most elaborate patterns, and appear to be furnished with raised teeth, like a file. In the Nudibranchiata the jaws are of great size and beauty of ornamentation (Fig. [109]).
Fig. 108.—Jaws of A, Triton australis Lam., Sydney; B, Ampullaria fasciata Reeve, Demerara; C, Calliostoma punctulatum Mart., New Zealand; D, Cyclophorus atramentarius Sowb., Sanghir; all × 15.
Fig. 109.—Jaws of A, Chromodoris gracilis Iher., × 15; B, Scyllaea pelagica L., × 7; C, Pleurobranchus plumula Mont., × 10; D, Pleurobranchaea Meckelii Lam., × 5/2.
The carnivorous genera, whether marine (e.g. Conus, Murex, Buccinum, Nassa) or land (e.g. Testacella, Glandina, Streptaxis, Ennea), are entirely destitute of jaws, the reason probably being that in all these cases the teeth of the radula are sufficiently powerful to do the work of tearing up the food without the aid of a masticatory organ as well. Jaws are also wanting in the Heteropoda, and in many of the Nudibranchiata and Tectibranchiata.