Page 354 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 341

mantle flap inhalent patelliform
penis siphon discoidal

mantle flap
convolute bi-conical

siphonal canal
head
shell operculum
eye opercular lobe
tentacle foot turbinate
proboscis digitate turreted pupiform
mouth
(a) Figure 13.12 Gastropod shell shapes.

spire growth line
body whorl
callus the opposite sense of coiling. The shell surface
whorl over operculum
umbilicus is commonly modified by strong growth lines,

varices ribs, tubercles and projections. Many gastro-
pods have an operculum covering the
spiral ornament aperture.
inductura on apex Gastropods developed a variety of shell
inner (= shapes. Eight different morphologies ranging
columellar) suture
lip from the simple patelliform to the complex
aperture
digitate shell are illustrated in Fig. 13.12 as a
selenizone
siphonal sample of the large amount of exoskeletal
canal variation in the group.
slit
Main gastropod groups and their ecology
growth line
Gastropods have been divided into three
umbilicus classes largely based on information from
axis of coiling
(b) their soft parts. Three subclasses are tradi-

tionally defined on the basis of the radula and
planispiral hyperstrophic their respiratory and nervous systems,
exhalent notch
although some of the groups may not be truly
monophyletic: (i) the Prosobranchia are fully
torted with one or two gills, an anterior
aperture mantle cavity and cap-shaped or conispiral
sinistral dextral
denticle shells; (ii) the Opisthobranchia are untorted
(having gone through torsion followed by
detorsion) with the shell reduced or absent,
and the mantle cavity posterior or absent; and
inhalent conisprial
siphonal (iii) the Pulmonata are untorted with the
notch mantle cavity modified as a lung, and the

(c)
shells are usually conispiral. Fossil taxa are
Figure 13.11 Gastropod morphology: usually assigned to these categories on the
(a) annotated reconstruction of a living basis of similarities in shell morphology with
gastropod, (b) annotated shell morphology of their living representatives.
three gastropod shell morphotypes, and (c) main The prosobranchs are mainly part of the
types of gastropod coiling strategy. marine benthos with a few freshwater and
terrestrial taxa. The primitive members of the
group, the Eogastropoda, are marine, mainly