Important families of mollusks
The Mollusca form one of the largest of the main divisions, or Phyla, of the Animal Kingdom, comprising over 60,000 living species. Fortunately but few need concern us from a medical or veterinary viewpoint. Up to the present not more than 60 species have been incriminated or even merely suspected as hosts of worm diseases. The difficulty, however, is to recognize these few species among the hundreds of others that are entirely harmless.
It does not appear feasible to present an account of molluscan intermediary hosts that might enable the layman or amateur to correctly name any species found to harbor the larval stages of a pathogenic worm. The greatest possible exactness, of course, must be reached in the identification of the host, if this information is to be of practical value. In the following notes, my main purpose is to convey to parasitologists a general idea of the classification, distribution, and habits of the mollusks they may have to deal with.