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Inquiry Questions

None for this chapter.

Self Test

1). Of the mollusks, snails are in the class of
    a). gastropods.
    b). bivalves.
    c). cephalopods.
    d). chitons.
Answer: a

2). A mantle is
    a). present only in bivalves.
    b). a structure that acts as a lung or contains gills.
    c). a rasping, tonguelike organ in mollusks.
    d). necessary for mollusks to be motile.
Answer: b

3). Segmentation was first apparent in the
    a). flatworms.
    b). annelids.
    c). mollusks.
    d). arthropods.
Answer: b

4). Which of the following is not present in polychaetes?
    a). a coelom
    b). parapodia
    c). permanent gonads
    d). setae
Answer: c

5). The lophophore, the structure characteristic of lophophorates,
    a). functions in gas exchange.
    b). functions in feeding.
    c). can be withdrawn when the animal is disturbed.
    d). All of these are correct.
Answer: d

6). The phylum that shows the greatest diversity, or the greatest number of species, is
    a). Arthropoda.
    b). Brachiopoda.
    c). Echinodermata.
    d). Mollusca.
Answer: a

7). Air for respiration enters the insect body through the
    a). tracheae.
    b). spiracles.
    c). tracheoles.
    d). Malpighian tubules.
Answer: b

8). Arthropods shed their old exoskeleton as they grow in a process known as
    a). tagmatization.
    b). metamorphosis.
    c). chrysalis.
    d). ecdysis.
Answer: d

9). Which animal group has radial symmetry, a water-vascular system, moves with tube feet, and has an endoskeleton?
    a). arachnids
    b). crustaceans
    c). echinoderms
    d). cnidarians
Answer: c

10). The echinoderms that lack distinct arms are the
    a). brittle stars.
    b). sea urchins.
    c). sea stars.
    d). Asteroidea.
Answer: b

Test Your Visual Understanding

1). The radula shown in the figure above is a feeding structure found in individuals of the phylum Mollusca but is lacking in one group of mollusks. What group of mollusks does not have a radula? How do individuals in this group eat?
Answer: The bivalves, class Bivalvia, do not have radulas. The radula is a rough, tonguelike feeding structure that is scrapped across the substrate or burrowed into prey. The bivalves are filter feeders, meaning that they feed off of small organisms trapped in their mantle cavity as water is passed through.

2). Horticultural oils are sometimes used as insecticides to eliminate insect pests from foliage by coating the insect with oil. Referring to the figure above, can you explain how this method of insecticidal control works?
Answer: The insect "breathes" through small openings in the surface of the body known as spiracles. Air passes into the spiracle and throughout the body by means of a network of tubules (trachea and tracheoles). By coating the insect body with horticultural oil, the spiracles are plugged up and air cannot pass into the body. Without air coming in through the spiracles, the insect suffocates.

Apply Your Knowledge

1). Freshwater bivalves are an important ecological resource because they filter freshwater systems. A population of freshwater bivalves located in a meter-squared area of substratum filters 10 m3 of water a day. How many liters of water are filtered per day (1 m3 equals 1000 liters)? How many liters of water are filtered per hour? How many liters of water would be filtered by a population filling a 5 m2 area of substratum?
Answer: If 1 m3 equals 1,000 liters, then 10 m3 would be 10,000 liters and so this population of bivalves would filter 10,000 liters per day. At 10,000 liters per day they would filter 10,000 / 24 hours or 416.7 liters per hour. If a 1 m3 population filters 10,000 liters of water a day, a population 5 m3 would filter 50,000 liters of water in a day.

2). Scientists believe the ancestral mollusk had a very limited shell, consisting mainly of calcareous plates. The shell became more developed in some groups but was lost in others. What is the evolutionary advantage of having a shell? Of not having one?
Answer: A mollusk's shell provides the animal with protection. A snail can pull into its shell and a clam can close its shell when threatened. However, a shell also limits mobility. Mollusks such as the cephalopods have essentially lost their shells over time. The cephalopods are predatory animals and as such, mobility is extremely important. The mobility that comes without a shell allows them to catch prey and escape attack if threatened.

3). Although arthropods are very successful in aquatic environments, what are the key adaptations that facilitated the invasion of the land by arthropods?
Answer: The arthropods successfully invaded land with the adaptations of an exoskeleton, jointed appendages, a respiratory system, and specialized structures such as compound eyes and wings. The exoskeleton increased mobility over the hydrostatic skeleton of the worms because it allowed a more solid structure for the anchoring of muscles. This allowed for the movement of individual appendages that increased speed and agility. The jointed appendages also increase agility. The respiratory system in insects (spiracles and tubules) and arachnids (book lungs) increased gas exchange in animals that were encased in a gas-impermeable exoskeleton and reduced water loss. The adaptation of specialized structures such as compound eyes and wings allowed terrestrial arthropods to better perceive and respond to changes in their environment.

4). Why is it believed that echinoderms and chordates, which are so dissimilar, are members of the same evolutionary line?
Answer: Two key evolutionary adaptations link these two animal lines together: deuterostome embryological development and an endoskeleton. No other animal group has these two features. In fact, a main reason why the echinoderms seem so different from the chordates is the radial symmetry that is present in the echinoderms. The larval stage of the echinoderms is bilaterally symmetrical and it is believed that the radial symmetry is a secondary adaptation to an aquatic environment.

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