METAZOA MOLLUSCA. 219 



many existing species, gave rise to different branches, 

 each one of which has passed or is passing through a 

 straight, a loosely coiled, and a tightly coiled stage. In 

 such a case we should have many similar forms which are 

 not closely related genetically but which are examples of 

 parallelism. In a natural classification these hold their 

 rightful position between the primitive and the more 

 specialized members of their respective groups. 



Such a classification is not based upon the structure of 

 the shell exclusively, inasmuch as the shell is a tell-tale part 

 of the organism ; generally speaking, a straight shell has 

 a straight body within it, and a coiled shell a coiled body. 

 The straight body has the bilateral symmetry character- 

 istic of early stages of animals, while the coiled body has 

 succeeded in making such a twist in its organs as to bring 

 the opening of the alimentary canal at the anterior instead 

 of the posterior end. This twisting is indicative of com- 

 plexity, and removes the form possessing it from its primi- 

 tive ancestor. We shall illustrate the possible evolution 

 of the Gastropod organism from straight through loosely 

 to tightly coiled and involute forms. Future research will 

 not probably change the principle of classification, though 

 it will doubtless bring the special forms selected into 

 genetic relations, so that each will hold its proper place 

 in its own particular series. We can readily conceive of 

 a straight tube becoming coiled so that the whorls would 

 barely touch one another. In this case the resultant form 

 would be a smooth unornamented spiral. In the loosely 

 coiled shell of to-day (he margin of the aperture has a 

 reflected lip as in Scala (No. 447, S. pretiosa Linn.). 

 This shell shows how a tube may be twisted so that the 

 whorls scarcely come in contact. The first whorl (No. 

 447) is nearly smooth, but each successive whorl shows an 

 increase in the size of the ridges till they look like pro- 

 jecting shelves extending entirely around the tube. By 

 examining the opening, it is seen that a shelf is in reality 

 the lip of the aperture which is made by the mantle during 

 a period of rest. 



