Animals show enormous diversity in size, habitat, habit and appearance. If features such as level of organisation, symmetry, coelom, segmentation, notochord and body plan are ignored, unrelated animals may be grouped together by superficial similarity and related animals may be separated. Fundamental characters make classification stable and biologically meaningful.
Without fundamental features, classification would be confusing, artificial and unable to show real relationships.
First note level of organisation, symmetry, germ layers, body cavity, segmentation, notochord, appendages, covering, digestive/circulatory/respiratory features and reproduction. Then compare these characters with taxonomic keys or descriptions and assign the specimen to the appropriate group.
Observe its fundamental body features, compare them with diagnostic characters and place it progressively into phylum/class.
Coelom is the body cavity lined by mesoderm. Its presence, absence or type reflects body organisation and evolutionary complexity. For example, Platyhelminthes are acoelomate, Aschelminthes are pseudocoelomate, and annelids, arthropods, molluscs, echinoderms and chordates are coelomate.
The nature of body cavity is very useful because it separates animals into acoelomates, pseudocoelomates and coelomates.
In intracellular digestion, food particles are engulfed and digested within food vacuoles, as in many simple animals. In extracellular digestion, enzymes are secreted into a cavity or alimentary canal where food is broken down before absorption, as in animals with more advanced organisation.
Intracellular digestion occurs inside cells; extracellular digestion occurs in a digestive cavity or tract outside individual cells.
Direct development lacks a free larval stage and the young gradually grows into the adult form. Indirect development includes a larva that differs from the adult and undergoes metamorphosis.
In direct development, the young resembles the adult; in indirect development, a larval stage occurs.
Parasitic flatworms are adapted for attachment and survival inside hosts. They may have suckers, hooks, reduced sensory/locomotory organs, resistant coverings and prolific reproduction. Examples include tapeworms and liver flukes.
Parasitic platyhelminths have hooks and suckers, a protective body covering, high reproductive capacity and often complex life cycles.
The exoskeleton protects the body and reduces water loss. Jointed appendages are modified for walking, swimming, feeding, sensing and reproduction. Many arthropods have wings, high reproductive capacity, varied feeding habits and strong sensory systems. These features let them occupy terrestrial, aquatic and aerial habitats.
Arthropods are highly successful because of their chitinous exoskeleton, jointed appendages, segmentation, efficient organs and adaptation to many habitats.
- a. Porifera
- b. Ctenophora
- c. Echinodermata
- d. Chordata
The water vascular system is the most distinctive feature of echinoderms and helps in locomotion, food capture, food transport and respiration.
(c) Echinodermata.
Chordates have a notochord, dorsal hollow nerve cord, pharyngeal gill slits and post-anal tail at some stage. Vertebrates are chordates in which the embryonic notochord is replaced by a cartilaginous or bony vertebral column. Protochordates such as Urochordata and Cephalochordata are chordates but not vertebrates.
All vertebrates possess chordate characters, but some chordates lack a vertebral column.
Osteichthyes possess an air bladder. Chondrichthyes lack it and therefore must swim constantly to avoid sinking. Thus, the air bladder is important for buoyancy control and energy saving.
The air bladder regulates buoyancy in bony fishes and helps them stay at different depths without constant swimming.
Forelimbs are modified into wings. Long bones are hollow and light. Feathers provide lift and insulation. Air sacs supplement lungs and improve respiration. The body is streamlined and the skeleton is fully ossified but light, supporting flight.
Birds have feathers, wings, pneumatic bones, a streamlined body, air sacs and strong flight muscles.
Eggs laid outside the body face greater environmental risk and mortality, so oviparous animals often produce many eggs. Viviparous embryos develop inside the mother and receive more protection and nourishment, so fewer young are produced.
Usually no; oviparous animals generally produce more eggs, while viviparous animals produce fewer young.
- a. Platyhelminthes
- b. Aschelminthes
- c. Annelida
- d. Arthropoda
True metameric segmentation first appears in annelids. Arthropods are also segmented, but they appear later in the sequence.
(c) Annelida.
Operculum covers gills in bony fishes, Osteichthyes. Parapodia occur in annelids. Scales are prominent in reptiles. Comb plates are characteristic of ctenophores. Radula occurs in molluscs. Hairs are mammalian. Choanocytes are characteristic of Porifera. Gill slits occur in Cyclostomata and Chondrichthyes.
(a)-(viii), (b)-(v), (c)-(iv), (d)-(i), (e)-(ii), (f)-(vii), (g)-(iii), (h)-(vi).
Human parasites occur across groups: flatworms such as Taenia, roundworms such as Ascaris and Wuchereria, arthropods such as lice, and protozoans such as Plasmodium. The list can be expanded with local examples studied in class.
Examples include Taenia, Fasciola, Ascaris, Wuchereria, Ancylostoma, Enterobius, Pediculus and Plasmodium.