CHITONID &. 243 
surella can scarcely be withdrawn within the shell, studded 
alike in both with papille and rugosities, and the same mar- 
ginal fringes. The nervous masses in the two accord closely: 
the only exception is the striking, but really unimportant, 
division of the cone of the Chitons into segments; but this 
incident may be accounted for on very simple grounds. We 
consider the fracture of the shell not a character denoting an 
affinity with the Annelida, as the body of the animal has no 
corresponding articulations, but simply an aid to facilitate 
locomotion. The foot of the strict symmetrical testaceous 
Gasteropoda is generally nearly concurrent with the length 
of the body, and forms its base, from which, by a gradually 
increasing cone, the animal becomes at maturity fixed to the 
summit of the shell by a powerful muscle: this structure 
united to an entire cone, and combined, as in the Bivalves, 
with the depressing effects of a complete hermaphroditism, 
that of Venus sine concubitu, almost extinguishes the loco- 
motive functions ; and we see throughout the Patelloid tribe, 
all of which have a foot co-extensive with the body, an almost 
entire fixation to the same spot; mdeed we believe that some 
of the Patelle pass their existence in the depressions of the 
rock on which they are cast as embryos, and the same apathy 
exists in Pileopsis and Calyptrea. But nature has judged fit 
to give the Chitons additional motive powers, for when fixed 
on the area of a smooth pebble, they will travel off it, whilst 
the Patelle are immoveable: this is owing to the integrity of 
the cone, and the enormous muscle which fixes the animal to 
its apex. This structure does not permit the body sufficient 
flexibility for much progression, and almost confines it to a 
vertical elevation and depression; but the Chitons, by the 
segmental condition of their shells, have accorded to them 
sufficient flexibility to obtain a vermicular motion, and its 
consequence, a greater facility of march. The Trachelipoda 
have infinitely superior attributes for motion, from the foot 
being fixed to a small portion of the body by an elastic 
cylindrical pedicle that affords a perfect pliability, and the 
acquisition of the necessary undulatory quality to effect a 
comparatively facile progression. 
R 2 
