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. flexibility, so the animal can crawl or burrow more readily
  2. the opportunity for the internal organs to grow and move independently of the external body wall
  3. a cushion for the internal organs if the animal receives a blow
  4. a hydrostatic skeleton if the animal is soft-bodied
  5. 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. bilateral symmetry
  2. protostome development and coelom
  3. circulatory system open
  4. the body is covered by a thick fold of tissue called the mantle, which forms a mantle cavity enclosing the gills and the anus
  5. the mantle commonly secretes a shell
  6. the main feeding appendage is a rasping radula
  7. 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. water flows through the mantle cavity in a siphon, which can provide jet propulsion
  2. the streamlined body shape, which may be a response to competition or predatory threat from bony fish
  3. tentacles for prey capture
  4. secretions of the salivary glands that may be poisonous to subdue prey
  5. the circulatory system, which is closed to provide efficient gas exchange to meet the demands of an active lifestyle
  6. ink sacs that can be emptied as a defence to hide the cephalopod or confuse the sense organs of a predator
  7. 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. exoskeleton providing structural support
  2. internal respiration via spiracles and branching tracheae reducing respiratory water loss
  3. internal fertilisation, with either direct or indirect transfer of sperm
  4. waxy cuticle that reduces water loss via the body surface
  5. the Malpighian tubules used as excretory organs minimising water loss in excretion
  6. 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
  7. 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.