
Figure 1. - Representatives of the mollusk classes, 
A. Neopilina, dorsal and ventral views, MONO- 
PLACOPHORA; B. chiton, AMPHINEURA; 
C, snail, slug, and limpet, GASTROPODA; D, tusk 
shell, SCAPHOPODA; E, clam, PELECYPODA; 
F, squid, CEPHALOPODA. 
The Class Monoplacophora includes only two living species, both found in 
abyssal marine waters. The saucer-shaped, limpet-like shell covers a 
bilaterally symmetrical animal with a mouth in front and anus behind. The 
round ventral foot is surrounded by five pairs of gills. The segmentation 
exhibited by these primitive mollusks suggests affinities with annelid 
worms and perhaps also with the arthropods. The radular teeth demonstrate 
relationships with the chitons. 
The Amphineura is another small primitive marine group which comprises 
the chitons (polyplacophorans) and a series of deep-water, wormlike, shell- 
less forms (aplacophorans), The shell of the chitons is divided into eight 
transverse calcareous plates (the only suggestion of segmentation) that 
cover the back. The foot is similar tothat ofthe monoplacophorans (but 
more elongate) and also of the gastropods. The aplacophorans are covered 
by a mantle bearing minute calcareous spicules. 
