Phylum Mollusca: Monoplacophora & Gastropoda

Question 1

Monoplacophora Body Plan (4)

Answer 1

Flattened shell
Multiple (8) ctenidia
Multiple dorso-ventral shell attached muscles
5-7 pairs of metanephridia

Question 2

Gastropoda body plan (4)

Answer 2

Visceropallial coiling
visceropallial elongation
Asymmetrical coiling
The central axis around which it coils (columella)

Question 3

Torsion

Answer 3

The torsion of the shell is the evolution from the monoplacophoran ancestor to the gastropod life form
Caused reduction of shell aperture, # of shell muscles (single pair) and # of ctenidia (single pair)
Posterior mantle cavity became anterior

Question 4

Rotation Hypothesis and 2 pieces of evidence

Answer 4

180-degree rotation from the hypothetical intermediate to form a present gastropod
Causes cross-over of nerve cords
The anatomy of living gastropods and the development of basal gastropods

Question 5

Adaptive Value (2)

Answer 5

Rotation for sensory epithelia (osphradia) to be towards the anterior end
Larval stage with mantel cavity near the velum would have higher survivability (some experiments do not show support for this)

Question 6

Fouling Problem - solutions (2)

Answer 6

Shell perforations for exhalent current (few do this)
Loss of right ctenidium, shift anus to the right side, oblique current and sometimes have a siphon (this is the solution for most gastropods)

Question 7

An alternative hypothesis for body plan

Answer 7

Bilateral mantle cavity
Unilateral loss
Subsequent remodelling to expand the mantle cavity
This causes cross over of the osphradia