MOLLUSKS 307 



to choose this field of natural history for special study. But the 

 true student of zoology does not confine his investigations in this 

 phylum to considerations of the shell only, for the protective 

 armor constitutes only a portion, although an important portion, 

 of the anatomy of the mollusk. On the same principle, it would 

 hardly suffice to examine only the skeleton of vertebrate animals 

 in order to become acquainted with the fishes or mammals. In a 

 sense, the testaceous covering of a mollusk may be likened to an 

 outer skeleton. 



The older zoologists were inclined to ignore almost entirely 

 the animal or "soft parts" of mollusks, and their classification 

 of the phylum into "univalve," "bivalve," and " multivalve," 

 based upon shell-characters alone, has since been proved to be 

 wholly artificial. The terms "univalve" and "bivalve" are, of 

 course, often and correctly used to-day to designate mollusks 

 possessing a single or a double shell, such as the common garden- 

 snail, which carries upon its back a single spiral shell, and, on 

 the other hand, the oyster or clam, which is inclosed within two 

 valves which fit closely together. But these terms are not 

 technically used, although they do indicate in a general way the 

 more modern classification. Even among some comparatively 

 recent conchologists a tendency to belittle the value of the 

 " slimy creature " within the shell has led to some curious errors 

 of grouping mistakes which more thorough and scientific in- 

 vestigation has brought to light. In general, the higher classi- 

 fication into classes, orders, suborders, and families is exclusively 

 based upon anatomical features, and to a large extent, also, is 

 that of the genera; but species are always determined by their 

 shell or conchological characteristics. 



When a certain familiarity with the forms and general ap- 

 pearance of shells is gained, a glance at the shell alone will almost 

 always suffice to place it at once in its true generic position, for, 

 along with modifications of the animal itself, corresponding modi- 

 fications of the shell are quite certain to take place. Hence a 

 fairly expert zoologist should be able to determine with con- 

 siderable accuracy from an empty and dead shell the anatomical 

 features of the animal that at one time inhabited it. 



