NEED TO CLASSIFY—BATTEN 513 
As we know today, there are several families of gastropods having 
representatives adapted to life in such rough and rocky environ- 
ments (fig. 6). All of them possess shells that are superficially quite 
similar, since they share a common habitat. Early zoological classi- 
fiers, looking at these cap-shaped shells, assumed that these different 
gastropods were members of the same group. The paleontologists, too, 
when they began to turn up such shells in the fossil record, classified all 
the cap-shaped shells as members of the limpet group. The classifica- 
tion, as formed by them, showed one group of “limpets” from very 
early geologic time to recent times. Such a classification would look 
like the illustration shown in figure 4. 
Meanwhile, the biologists—who were studying living lmpets— 
soon recognized that, in addition to the “true” ones (which they called 
patellids), there actually were several other more or less distantly 
related families of gastropods, members of which resembled true 
limpets. 
This discovery was possible because the biologists studied the living 
tissue and organs. Unfortunately, the paleontologists had only the 
shells available, and were unable to study the differences in the organs 
between the various cap-shaped forms. For many years, in conse- 
quence, little change occurred in classification of the extinct forms. 
Before we go further, we must learn something more about the 
gastropods. Most organisms, we know, possess some sort of sym- 
metry in their bodily arrangement. The commonest type of symmetry 
is a bilateral arrangement, in which one side of the organism is a 
mirror image of the other, and the organism’s head and tail le at 
opposite ends of the body. Most gastropods are asymmetrical, having 
lost one “side” sometime in the course of their evolution. When we 
look at a snail, we see that the soft parts of its body are contained in 
a shell which, although often coiled, is usually a long, narrow cone, 
open at one end (see fig. 3). 
An examination of rock-clinging patellids—the true limpets—shows 
that, while they have cap-shaped rather than long, narrow shells, here, 
too, only one “side” of the organism is present and anus and mouth are 
in close proximity. In other words, all the limpets are typical, coiled 
asymmetrical gastropods (see fig. 5). 
We have already been introduced to another class in the order of 
mollusks—the polyplacophorans, or chitons. These chitons share a 
rock-clinging environment with the limpets, but they are a much 
more primitive form of mollusk. Anus and mouth are at opposite 
ends of the chiton’s body and the body itself is bilaterally symmetrical. 
The chitons’ shells are different from those of the patellids, having 
eight separate plates instead of a single shell (see fig. 9). It was 
thus obvious—even to the early classifiers—that, while these two 
