Chapter 4: Animal Kingdom (Diversity in the Living World)

Without fundamental features (level of organisation, symmetry, germ layers, coelom, segmentation, notochord, organ systems) we would have to depend on superficial features such as size, colour, shape or habitat, which vary a lot. Then (i) unrelated animals would be grouped together (e.g. whale with fishes, bat with birds) and related ones separated; (ii) the millions of species could not be arranged into a workable hierarchy; (iii) a new species could not be given a definite position; (iv) evolutionary relationships would not be reflected.

Examine it for the fundamental features, step by step: (i) level of organisation: cellular, tissue, organ or organ system; (ii) symmetry: asymmetrical, radial or bilateral; (iii) germ layers: diploblastic or triploblastic; (iv) body cavity: acoelomate, pseudocoelomate or coelomate; (v) segmentation; (vi) notochord (chordate or non-chordate); then the special features of the phylum (e.g. jointed legs, water vascular system), the class features (e.g. scales, feathers, hair, number of heart chambers), and so on down to the genus and species, and finally give it its binomial name.

The presence and nature of the body cavity divides triploblastic animals into three clear groups: acoelomates (no cavity: Platyhelminthes), pseudocoelomates (cavity not lined by mesoderm: Aschelminthes) and coelomates (true coelom lined by mesoderm: Annelida to Chordata). The coelom gives space for organs to develop and move, cushions them, and allows more complex organ systems; so it reflects the evolutionary level of the animal and is a basic criterion of classification.

In direct development the young ones resemble the adult and grow into adults without a larval stage (e.g. reptiles, birds, mammals, some roundworms). In indirect development there is a larval stage, morphologically distinct from the adult, which changes into the adult by metamorphosis (e.g. sponges, flatworms, frogs, many insects, echinoderms).

(i) Dorsoventrally flattened body; (ii) hooks and suckers for attachment to the host; (iii) they absorb nutrients directly from the host through the body surface (the digestive system is reduced or absent, e.g. tapeworm); (iv) flame cells for excretion; (v) hermaphrodite with internal fertilisation and many larval stages, often needing more than one host. Examples: Taenia, Fasciola.

(i) A chitinous exoskeleton protects them and prevents water loss, allowing life in every habitat; (ii) jointed appendages are modified for walking, swimming, flying, feeding and sensing; (iii) efficient respiratory organs (tracheae, gills, book lungs) and Malpighian tubules for life on land and in water; (iv) many can fly (insects), so they spread widely and escape enemies; (v) small size, short life cycles and very high reproductive rates; (vi) metamorphosis lets larvae and adults use different food. So they have adapted to and occupied almost all habitats.

(c) Echinodermata.

Chordates are animals that have a notochord at some stage. In vertebrates the embryonic notochord is replaced by a cartilaginous or bony vertebral column in the adult, so every vertebrate is a chordate. But the protochordates (Urochordata, Cephalochordata) have a notochord (in the larval tail, or throughout life) that is never replaced by a vertebral column; they are chordates but not vertebrates.

The air bladder of bony fishes regulates buoyancy: by changing the amount of gas in it the fish can float at any depth without spending energy. Cartilaginous fishes, which lack an air bladder, must swim constantly to avoid sinking.

(i) Body covered with feathers, streamlined; (ii) forelimbs modified into wings; (iii) pneumatic long bones (hollow with air cavities) make the skeleton light but strong; (iv) lungs with air sacs give the large oxygen supply needed; (v) completely four-chambered heart and warm-bloodedness give high energy; (vi) a beak without heavy teeth, a crop and gizzard; (vii) the oil gland keeps feathers waterproof.

Usually not. An oviparous animal lays eggs that develop outside, unprotected; many are eaten or die, so it lays a large number of eggs (fish, frogs). A viviparous mother nourishes and protects the young inside her body and cares for them after birth, so their survival is high and she produces fewer young. (Survival chances, not just numbers, decide the strategy.)

(c) Annelida (metameric segmentation).

(a)–(viii), (b)–(v), (c)–(iv), (d)–(i), (e)–(ii), (f)–(vii), (g)–(iii), (h)–(vi).

Taenia solium (tapeworm) and Fasciola (liver fluke); Ascaris (roundworm), Wuchereria (filaria worm), Ancylostoma (hookworm), Enterobius (pinworm); Pediculus (head louse); leeches (Hirudinaria, blood-sucking); among protozoans, Plasmodium, Entamoeba and Trypanosoma.