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. 
Acmea testudinalis (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 
Acmea is an ancestral type just because the simple character of 
the shellis suggestive of the model chosen to represent a schematic 
mollusk ; anatomically Acmea 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 
