Phylum Mollusca: Body Plan and Signature Features
Molluscs form the second largest animal phylum, and their body architecture is one of the most recognisable in the invertebrate world. This quiz tests the essentials of Lesson 17: the molluscan organ-system level of organisation, bilateral symmetry, the triploblastic and coelomate condition, and the unsegmented body with its calcareous shell.
You will be asked about the head, muscular foot and visceral hump, the soft spongy mantle draped over the hump, and the mantle cavity with its feather-like gills that serve respiratory and excretory functions. The radula, the file-like rasping organ in the mouth, and the sensory tentacles of the anterior head region also appear.
- Habitat range: terrestrial or aquatic (marine and fresh water)
- Body plan: unsegmented, shell-covered, soft-bodied beneath
- Key structures: mantle, mantle cavity, gills, radula
- Representative genera: Pila, Pinctada, Sepia, Loligo, Octopus, Aplysia, Dentalium, Chaetopleura
Work through all fifteen questions carefully and read each explanation, since the contrasts with arthropods and echinoderms are exam favourites.
Q1. In terms of the number of named species, Mollusca ranks second in the animal kingdom. Which phylum ranks first?
Mollusca is the second largest animal phylum, surpassed in species count only by Arthropoda, the largest phylum of Animalia.
Q2. Which statement correctly describes the habitat range of molluscs?
Molluscs are terrestrial or aquatic, and the aquatic forms occur in both marine and fresh water.
Q3. The level of organisation shown by molluscs is best described as:
Molluscs have an organ-system level of organisation, the highest grade among the invertebrates surveyed in this course.
Q4. Which combination correctly states the symmetry and germ-layer condition of molluscs?
Molluscs are bilaterally symmetrical, triploblastic and coelomate animals.
Q5. A key feature that distinguishes the molluscan body from that of an arthropod is that the molluscan body is:
The molluscan body is unsegmented and covered by a calcareous shell, whereas arthropods are segmented with a chitinous exoskeleton.
Q6. Which set of structures forms the basic architectural signature of a mollusc?
The molluscan body has a distinct head, a muscular foot and a visceral hump, all covered by a calcareous shell.
Q7. The soft and spongy layer of skin that forms a covering over the visceral hump is called the:
A soft and spongy layer of skin forms a mantle over the visceral hump.
Q8. The mantle cavity of a mollusc is best defined as:
The space between the hump and the mantle is called the mantle cavity, and it houses the feather-like gills.
Q9. In molluscs, the feather-like gills present in the mantle cavity perform which functions?
The feather-like gills in the mantle cavity have respiratory and excretory functions.
Q10. The file-like rasping organ used for feeding in molluscs is the:
The mouth contains a file-like rasping organ for feeding called the radula.
Q11. Where in the molluscan body are the sensory tentacles located?
The anterior head region of a mollusc has sensory tentacles.
Q12. Which of the following is a correct pairing of a molluscan genus with its common name?
Pinctada is the pearl oyster. Sepia is the cuttlefish, Loligo is the squid, and Aplysia is the sea-hare.
Q13. Which genus listed among the molluscan examples is commonly called the devil fish?
Octopus is commonly called the devil fish. Dentalium is the tusk shell, Chaetopleura the chiton and Pila the apple snail.
Q14. Regarding reproduction, molluscs are usually:
Molluscs are usually dioecious and oviparous with indirect development.
Q15. Which feature of molluscs contrasts most directly with the adult condition of echinoderms?
Molluscs retain bilateral symmetry in the adult and have a well-developed excretory arrangement within the mantle cavity, whereas adult echinoderms are radially symmetrical and have no excretory system.