AY Honors/Shells - Advanced/Answer Key

2. Define the term "mollusca" or "mollusk."

The word mollusc is derived from the French mollusque, which originated from the Latin molluscus, meaning thin-shelled, from mollis, soft

The body of a mollusk consists of three sections; a head, with eyes or tentacles; a muscular foot; and a visceral mass housing the organs.

3. Learn the classification terms of mollusks, know the distinguishing characteristics of each, and become acquainted with several species under each class.

There are ten classes of molluscs; eight of the classes have living representatives, the other two classes are known only from fossils. More than 250,000 species of mollusc are recognized and named. Snails (Gastropoda) account for about 80% of living mollusc diversity.

Class	Major organisms	Extant species	Distribution
Caudofoveata	worm-like organisms	70	deep ocean
Aplacophora	solenogasters, worm-like organisms	250	deep ocean
Polyplacophora	chitons	600	rocky marine shorelines
Monoplacophora	limpet-like organisms	11	deep ocean
Gastropoda	abalone, limpets, conch, nudibranchs, sea hares, sea butterfly, snails, slugs	150,000	marine, freshwater, land
Cephalopoda	squid, octopus, cuttlefish, nautilus	786	marine
Bivalvia	clams, oysters, scallops, mussels	8000	marine, freshwater
Scaphopoda	tusk shells	350	marine
Rostroconchia †	fossils; probable ancestors of bivalves	extinct	—
Helcionelloida †	fossils; snail-like organisms such as Latouchella	extinct	—

4 Distinguish between univalve and bivalve mollusks from the following considerations:

a. Shell

The terms univalve and bivalve refer to the configuration of the shells of the mollusks. Univalves, such as snails, have shells consisting of one (thus uni-) piece. Bivalves, such as mussels, have shells consisting of two (thus bi-) pieces hinged together.

b. Mollusk body

Univalves

Univalve bodies could almost be described as folded such that the mouth and anus both appear the the shell's opening.

Bivalves

Bivalves on the other hand, have flattened bodies.

c. Reproduction

Univalves

All land snails are hermaphrodites, producing both spermatozoa and ova. In other words, each individual is both male and female. Some freshwater snails, such as Apple Snails, and marine species, such as periwinkles, have separate sexes; they are male and female. Most snails can mate when they are around 1 year old.

Prior to reproduction, most land snails perform a ritual courtship before mating. This may last anywhere between two and twelve hours. Prolific breeders, pulmonate land snails inseminate each other in pairs to internally fertilize their ova. Each brood may consist of up to 100 eggs.

Pulmonate land snails and slugs have a reproductive opening on one side of the body, near the front, through which the outer reproductive organs are extruded so that exchange of sperm can take place. After this, fertilization occurs and the eggs develop.

Garden snails bury their eggs in shallow topsoil primarily while the weather is warm and damp, usually 5 to 10 cm down, digging with their foot. Egg sizes differ between species, from a 3 mm diameter in the grove snail to a 6 cm diameter in the Giant African Land Snail. After 2 to 4 weeks of favorable weather, these eggs hatch and the young emerge. Snails may lay eggs as often as once a month.

Bivalves

In bivalves, the sexes are usually separate, but some hermaphroditism is known. Bivalves practice external fertilization. External fertilization is a form of fertilization in which a zilo cell is united with an egg cell external to the body of the female. Thus, the fertilization is said to occur "externally". This is distinct from internal fertilization where the union of the egg and sperm occur inside the female after insemination through copulation.

In sexual reproduction, there must be some way of getting the sperm to the egg. Since sperm are designed to be mobile in a watery environment, aquatic mollusks can make use of the water in which they live. Eggs and sperm are simultaneously shed into the water, and the sperm swim through the water to fertilize the egg in a process known as broadcast fertilization.

d. Movements

Snail climbing a blade of grass

Univalves

Most snails move by gliding along on their muscular foot, which is lubricated with mucus. This motion is powered by succeeding waves of muscular contraction which move down the undersurface of the foot. This muscular action is clearly visible when a snail is crawling on the glass of a window or aquarium. Snails move at a proverbially low speed (1 mm/s is a typical speed for adult Helix lucorum). They produce mucus in order to aid locomotion by reducing friction, and the mucus also helps reduce the snail's risk of mechanical injury from sharp objects. This means that they can 'walk' over sharp objects like razors without being injured.

Bivalves

Razor shells (Ensis spp.) can dig themselves into the sand with great speed to escape predation. Scallops can swim to escape an enemy, clapping their valves together to create a jet of water. Cockles can use their foot to leap from danger. However these methods can quickly exhaust the animal. In the razor shells the siphons can break off only to grow back later.

e. Securing food

Bivalves

Bivalves are unique among the molluscs for lacking a radula; they feed by siphoning and filtering large particles from water.

Univalves

Univalves include some that are herbivores, detritus feeders, predatory carnivores, scavengers, parasites, and also a few ciliary feeders, in which the radula is reduced or absent. The radula of a univalve is usually adapted to the food that a species eats. The simplest univalves are the limpets and abalones, herbivores that use their hard radulas to rasp at seaweeds on rocks. Many marine univalves are burrowers, and have soft siphons or tubes that extend from the mantle. Sometimes the shell has a siphonal canal to accommodate this structure. A siphon enables the animal to draw a small flow of water into their bodies. The siphon is used primarily to "taste" the water, in order to detect prey from a distance. Univalves with siphons tend to be either predators or scavengers.

f. Self-preservation

Univalves

When retracted into their shells, many snails with gills (including many marine, some freshwater and some terrestrial species) are able to protect themselves with a door-like anatomical structure called an operculum. Some snails hibernate during the winter (typically October through April in the Northern Hemisphere). They may also estivate in the summer in drought conditions. To stay moist during hibernation, a snail seals its shell opening with a dry layer of mucus called an epiphragm.

Bivalves

The radical structure of the bivalves affects their behaviour in several ways. the most significant is the use of the closely-fitting valves as a defence against predation and, in intertidal species such as mussels, drying out. The entire animal can be contained within the shell, which is held shut by the powerful adductor muscles. This defence is difficult to overcome except by specialist predators such as the Starfish and Oystercatcher. The file shells (Limidae) can produce a noxious secretion when threatened, and the fan shells of the same family have a unique, acid-producing organ.

8. Explain the activities of the shipworm.

Adventist Youth Honors Answer Book/Nature/Shipworm

10. Name two mollusks that have no shells.

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  Slug
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  Octopus
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  Cuttlefish
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  Squid