OBSERVATIONS ON THE FUNCTIONING OF THE ALIMENTARY SYSTEM OF THE SNAIL LYMNAEA STAGNALIS APPRESSA SAY

¹ Zoological Laboratory of the University of Wisconsin, Madison

1. A balanced physiological salt solution was developed which maintains contractions of the vas deferens for approximately 66 hours.

2. Cathepsin was found in greatest concentration in the liver and no activity could be ascertained in the gut fluids. Some trypsin was indicated in the salivary glands. Amylase showed greatest activity in the salivary glands and the liver.

3. Muscular activity of the alimentary system involves the manipulation of the mouth parts in the buccal mass, peristalsis in the remainder of the tract, marked pulsatory movements of the postesophagus, crop, pylorus and liver, and a kneading motion of the gizzard. The radula is moved principally by the action of the odontophore but also operates independently of it.

4. The entire alimentary system, with the exception of the gizzard and parts of the buccal cavity, is ciliated. The cilia show definite directional streams which function in propelling food particles, in sorting food and in consolidating fine refuse particles with the aid of mucoid substances.

5. Sand is consumed normally by the snail and is necessary for the proper functioning of the gizzard in the crushing of food particles. Very little trituration is performed by the mouth parts.

6. The pylorus is composed of a complicated system of folds and passages and counter ciliary currents and functions as a filter which permits only the soluble and the finer food particles to pass into the liver. It shunts the undigested residues from the gizzard into the prointestine.

7. In the liver the digestive cells function in secretion, assimilation, intracellular digestion and excretion. The indigestible foods and the excretory products, as variably shaped and colored inclusion bodies, are eliminated in vacuoles.

8. The cecum functions in collecting the finer residues from the liver and forces them in a continuous string into the prointestine.

9. The residual material coming from the gizzard, liver and cecum is characteristic for each organ and is readily identified as distinct in the fecal pellet.

10. The prointestine is specialized in the final consolidation of gizzard, liver and cecal strings with the aid of cementing substances secreted by the basophilic flask cells and the basal cells.

11. The rhythmic nature of the liver is disclosed principally by a study of the fecal pellets.

12. L. s. appressa is an herbivore. Food bits are cut away by the radula and swallowed. In the buccal cavity the food receives mucus from the buccal gland cells, mucous cells and the salivaries and enzymes from the latter. Temporary storage and initial digestion occur in the postesophagus. Digestive fluids pass up from the liver in the pulsatory movements of the stomach region which keep the fluid gut contents in constant circulation. The crop, gizzard and anterior portion of the retrocurrent passage of the pylorus comminute the food. Amebocytes present in the gut contents appear to aid in digestion. Soluble and fine particles of food pass through the pyloric filter into the liver where it is assimilated by the digestive cells. Assimilation also occurs in the pylorus and absorption possibly in the intestine. There is some evidence that the pulsatory movements of the stomach region cease during the passage of the gizzard and the liver strings.

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