348 MARINE INVERTEBRATES 



always perfectly smooth. When foreign substances, such as 

 grains of sand, enter the shell and cannot be removed by the 

 animal, the irritation caused thereby to the soft, fleshy creature 

 induces discharges of a liquid from the glandular surface of the 

 mantle, which hardens about the offending substance and glazes 

 it over with a smooth, pearly deposit. 



Acmcea testtidinalis (page 343) presents a special type of gastero- 

 pod shell which is found in several families. Here the spire 

 seems to be wholly absent, and the entire shell consists of but one 

 large body-whorl. There are very many of these patelliform 

 species, inhabiting many seas and belonging to many different 

 genera, and in nearly all cases their embryonic shells display a 

 spiral form. After birth the animal does not build his house 

 upon the spiral plan, but expands the shell into one large shield- 

 like covering. The student, however, must not presume that 

 Acmcea is an ancestral type just because the simple character of 

 the shell is suggestive of the model chosen to represent a schematic 

 mollusk ; anatomically Acmcea presents the complications of body- 

 torsion which show a very considerable evolutionary change, 

 and indicate that its simple shell is probably a degenerate form of a 

 once more highly developed and convoluted one. 



The forms, the architecture, and the painting of gasteropod 

 shells are so infinite in variety that it would be unwise to attempt 

 a description of their marvels. A close observer of nature's 

 works soon becomes prepared for every surprise, but he never 

 ceases to be charmed and fascinated by his new discoveries. The 

 careful student alone can learn really to see and appreciate the 

 wonders of nature, and this is especially true in the study of the 

 Mollusca. 



CLASSIFICATION OF GASTEROPODS 



The Gasteropoda far exceed all the other divisions of the Mol- 

 lusca in the number of their genera and species. Apparently 

 this has not always been the case. There is evidence tending to 

 show that in past geological epochs the pelecypods (the bivalve 

 shells) outnumbered the gasteropods, but that in the course of 

 time the increase in the genera of gasteropods has been more 



