Download ANSWERS TO REVIEW QUESTIONS – CHAPTER 39

ANSWERS TO REVIEW QUESTIONS – CHAPTER 39
1. Which animal phyla are coelomate protostomes? Explain what this means. (p. 955)
Coelomate animals have a body cavity called the coelom. The cellular lining of this cavity, the
peritoneum, is of mesodermal origin. The coelom may arise from solid masses of cells that form
between the digestive tube and the embryonic body wall (protostome development) or outpocketings of
the embryonic gut (deuterostome development). The annelids, arthropods and molluscs are the major
protostome phyla.
The development of some form of internal body cavity is an important step in the development of
higher animals. It confers many advantages such as:
1.
2.
3.
4.
5.
flexibility, so the animal can crawl or burrow more readily
the opportunity for the internal organs to grow and move independently of the external body wall
a cushion for the internal organs if the animal receives a blow
a hydrostatic skeleton if the animal is soft-bodied
assistance in circulation and waste disposal.
Small, thin animals such as flatworms cope well without a body cavity, but some form of cavity is
present in most animal phyla.
2.
You are collecting in a marine habitat and find an elongated animal with numerous bristles
on body segments. Each segment also bears a pair of lateral fleshy lobed appendages. Name
the phylum and the class to which this animal probably belongs. Name the appendage
described and state its function(s). (p. 958)
This animal is an annelid worm from the class Polychaeta. Polychaete lifestyles are varied and include
free-moving forms called errant polychaetes as well as sedentary species that burrow or construct tube
shelters. Polychaetes are commonly known as bristle worms because of the many chitinous bristles or
setae that occur on the lateral fleshy lobed appendages called parapodia. The parapodia are laterally
compressed, so the setae tend to spread out like a fan. A single cell at its base secretes each seta and
new ones are produced as older ones are lost.
The parapodia provide a large surface area for gas exchange and are often associated with gills, which
can be modified parts of each parapodium. In errant forms the parapodia also assist in locomotion. The
setae on the parapodia may have a defensive function and are poisonous in some species.
3.
Earthworms move by peristaltic locomotion. Describe what this means and how they do it.
Why is internal compartmentalisation important in the process? (p. 956)
Annelids such as earthworms use their coelomic fluid as a hydrostatic skeleton. The coelom itself is
divided by septa between the segments, so the animal can change the shape of each segment
individually, using its circular and longitudinal muscles. Rhythmic waves of muscle contractions called
peristalsis move from head to tail of the animal to enable movement. Contractions of the circular
muscles constrict and elongate the worm, while contractions of the longitudinal muscles thicken and
shorten it. In some segments, the worm’s bristles anchor it to the substrate and the longitudinal muscles
contract to thicken and shorten the body, while the circular muscles relax. In other segments the
circular muscles contract and the body elongates, while the longitudinal muscles relax and the bristles
release their hold on the substrate. The alternating waves of free and anchored segments allow the
worm to pull itself along.
4. Describe the basic body structure of the hypothetical general mollusc. (pp. 961–962)
The principal body features of molluscs are:
1.
2.
3.
bilateral symmetry
protostome development and coelom
circulatory system open
4.
5.
6.
7.
the body is covered by a thick fold of tissue called the mantle, which forms a mantle cavity
enclosing the gills and the anus
the mantle commonly secretes a shell
the main feeding appendage is a rasping radula
large, well-developed muscular foot.
5. Compare and contrast the method of feeding in a gastropod and a bivalve. (pp. 962–964)
Gastropods feed in a wide variety of ways including carnivory, herbivory and scavenging. They
employ their toothed radulas to grate, rasp and cut their food and then convey it to their mouths. In the
simplest form, food particles are rasped off and passed into a gutter in the centre of the radula for
collection and passing into the stomach. In some carnivorous forms the radula has strongly curved
spines to seize the prey and withdraw it into the gut.
By contrast, bivalves are filter feeders and lack a radula. Their gills are larger than required for gas
exchange alone and extend the length of the mantle cavity. They are covered with cilia and used to
filter food. Incoming water passes through the gills, fanned on its way by the beating cilia.
6.
What are the features of cephalopods that reflect their active, high-energy way of life?
(pp. 964–967)
Cephalopod features are related to their active, predatory lifestyle. Examples of these features include:
1.
2.
3.
4.
5.
6.
7.
water flows through the mantle cavity in a siphon, which can provide jet propulsion
the streamlined body shape, which may be a response to competition or predatory threat from
bony fish
tentacles for prey capture
secretions of the salivary glands that may be poisonous to subdue prey
the circulatory system, which is closed to provide efficient gas exchange to meet the demands of
an active lifestyle
ink sacs that can be emptied as a defence to hide the cephalopod or confuse the sense organs of a
predator
a well-developed brain with excellent sight, touch, memory and shape recognition.
7.
Arthropods have a hard, chitinous exoskeleton. What are the advantages and disadvantages
of having a hard skeleton external to the body? (pp. 970–971)
The rigid external exoskeleton consists of a series of plates connected with flexible articular
membranes to permit movement. Jointed appendages may be modified for movement, feeding,
reproduction or sensing. One striking example is the wings of insects, formed from thin, lightweight
outfoldings of the body wall supported by networks of veins. Flight assists insects in finding food or
new habitats, exploiting temporary resources, escaping predators, dispersing and exchanging genetic
material between populations. It is one of the main reasons for their extraordinary species diversity.
Advantages: It is primarily composed of chitin, a polysaccharide combining strength, flexibility and
light weight. In most terrestrial forms a waxy epicuticle on the outer surface reduces water loss. Many
crustaceans impregnate the exoskeleton with calcium salts for greater strength.
Disadvantages: The exoskeleton cannot expand, so for growth it is shed periodically in a process called
moulting or ecdysis. The arthropod is very vulnerable immediately after the old exoskeleton is shed
and the new one is not yet hardened.
8.
List the characteristics that would enable you to identify a nematode. In what way do they
cause problems for humans? (pp. 968–969)
Most nematodes have a characteristic bilaterally symmetrical, cylindrical shape tapering at both ends.
The outer body covering is a thick cuticle of protein. They have only longitudinal muscles, which
contract against a hydrostatic skeleton provided by high internal pressures. The mouth is often
accompanied by specialised lips and connects to a digestive tract that terminates in an anus. The body
cavity is a pseudocoelom. The nervous system consists of a ring of ganglia around the oesophagus and
ventral, dorsal and lateral nerves that run posteriorly. Nematodes may cause problems for humans by
parasitising people or domestic livestock. They also attack plants and may reduce the productivity of
crops or harm ornamental plants.
9.
Briefly describe the basic mouthparts of an insect and how they are modified in specialist
feeders such as a housefly and a female mosquito. (pp. 975–976)
Mouthparts may taste food, hold it or prepare it for ingestion. In insects that bite and chew, these
mouthparts consist of a pair of mandibles, a pair of maxillae (each of which carries a palp) and a
labium derived from a fused second pair of maxillae (and also bearing paired palps). A chitinous plate
called the labrum covers these, while the tongue-like hypopharynx lies behind the mouth. The
mandibles are heavy and toothed and can cut, crush, tear and chew food. The other mouthparts assist in
manipulating food for the action of the mandibles. Figure 39.19 shows this basic structure.
The basic biting form of mouthparts is modified for different foods in several insect orders. For
example, in butterflies and moths the maxillae are modified into a long tube used to suck liquid food
such as nectar from flowers. The tube is coiled when not in use and the other mouthparts are vestigial.
In biting flies, the mandibles are knife-like and produce a wound. Blood is collected using a spongelike
labium and drawn into the mouth by a tube formed by the hypopharynx. In houseflies, liquid food is
taken up by the labium with other mouthparts reduced. If the food is not liquid, saliva is extruded to
soften it before ingestion. Some of these mouthpart adaptations are illustrated in Figure 39.20.
10. List the features of insects that have allowed them to expand into terrestrial environments
and indicate why they are significant in this regard. (pp. 975–978)
The terrestrial environment presents problems of support because of the reduced buoyancy of air
relative to water, a need to regulate water loss and an inability to use water as a medium for
fertilisation. Insects possess several significant adaptations to overcome these difficulties, including:
1.
2.
3.
4.
5.
6.
7.
exoskeleton providing structural support
internal respiration via spiracles and branching tracheae reducing respiratory water loss
internal fertilisation, with either direct or indirect transfer of sperm
waxy cuticle that reduces water loss via the body surface
the Malpighian tubules used as excretory organs minimising water loss in excretion
complete metamorphosis (holometabolous development) in many orders confers several
advantages, including: larvae specialised for feeding and growth while adults are specialised for
dispersal and reproduction, no competition between larvae and adults and two potentially resistant
stages (egg and pupae) in the life cycle
flight, which allows insects to exploit temporary resources, disperse readily, escape from
predators, find new habitats and mix different populations at mating.
Insects also have significant co-evolutionary associations with flowering plants. Insect pollination is
often specific so that plants produce less pollen, the insect eating some pollen or nectar as a reward.