Crocodiles (Family Crocodylidae) and hippopotamus (Hippopotamus amphibius; Family Hippopotamidae) are two distinct species in the animal kingdom. The habitat of these animals misleads many people into thinking that they are amphibians. In addition to staying in water for longer hours, hippo has the term ?amphibius? as its species name (but notice the difference in spelling between the species name ?amphibius? and the English adjective ?amphibious?). The Latinized expression ?amphibius? meaning capable of living both on land and in water actually deludes many people into concluding that hippos are amphibians. In reality, a crocodile is a reptile and a hippopotamus is a mammal.
The origin of the ?generational error? described above is rooted in the early development of taxonomy. Thanks to modern biology, we know that living things are ?now? put into five main kingdoms namely Animalia (animals), Plantae (plants), Fungi (fungi), Prokaryotae (prokaryotes/bacteria) and Protoctista or Protista (protists). For examples, human and mosquito are animals, cocoa and cassava are plants, mushroom and toadstool are fungi, salmonella and rhizobia are bacteria, and amoeba and trypanosome are protists. No living thing belongs to two kingdoms.
However, organisms within each kingdom vary widely. For example, humans, goats, bats, whale and ants are all animals, yet all of them look different from one another. In consequence, biologists have established several criteria to classify organisms for easy identification and naming.
Many of these criteria are based on analogous traits (features organisms have in common because of the lifestyle or environment in which they live). For example, grasshoppers and bats both have wings to fly and may be classified as flying animals. Also, humans, ostrich and ants all have legs for walking and may be classified as walking animals. Finally, tilapia, whales and seals have fins/flippers for swimming and may be classified as swimming animals.
The features used for grouping the animals mentioned in the preceding paragraph are termed convergent evolution (development of analogous traits). These features hardly show similarity in many biological traits in the organisms. For example, bats have hair on the bodies and give birth to live babies, yet bats fly. That is, ability to fly shows little similarity among flying animals. Compare bat and dove. (A dove /dav/ has feathers and lays eggs.)
Thus, ambiguity occurs if organisms are to be classified mainly based on analogous traits. To narrow down these differences, Carolus Linnaeus (the Swedish naturalist) introduced and/or popularized the natural system of classification. The natural system is based on homologous traits (similarities among living things that descended from a common ancestor).
The natural system identifies seven ranks (hierarchies or taxa) of classification. These ranks are kingdom, phylum/division, class, order, family, genus and species. The names of any groups of organisms in the first six taxa are treated as proper nouns (eg, humans are in the kingdom Animalia, phylum Chordata, class Mammalia, order Primates, family Hominidae and genus Homo). Each scientific name for a group has a corresponding common name (eg, Chordata = chordates, Mammalia = mammals and Primates /praimeitiis/ = primates /praimeits/).
An organism that belongs to one specific group within any taxon cannot belong to another. We demonstrate this important principle using the taxonomic rank class: Some of the major classes in the kingdom Animalia are Mammalia (= mammals, eg, humans, whales, bats and hippos), Reptilia (= reptiles, eg, tortoises, snakes, lizards and crocodiles), Amphibia (= amphibians, eg, frog, toad, newt and salamander) and Aves (= avians or birds, eg, fowl, pigeon /pijin/, ostrich and penguin).
From the above, we may infer the unique features possessed by members of each of the classes.
a. Most mammals are viviparous (give birth to live offspring) and others are oviparous (lay eggs which hatch into the young ones, eg, anteater). However, all mammals breastfeed their young ones. Breastfeeding the young is the feature common to all mammals. They may live on land or in water or in both. For example, bats fly and live in trees but they are never birds. Hippos walk on land and spend the whole night and much of the day in water but only move to land at dawn to graze on plants. However, hippos are mammals, and not amphibians.
b. Reptiles are animals whose bodies are covered in thick scales. They reproduce by laying eggs which have thick leathery coverings. The eggs are buried in damp soils or organic matter. ?The eggs hatch by themselves?. Thus, crocodiles which live in water and usually come to land to bask (get sunlight) while gaping (opening their mouths wide) are neither fishes nor amphibians. Crocodiles are reptiles.
c. AMPHIBIANS ARE ANIMALS THAT LIVE ON LAND BUT RETURN TO WATER TO PRODUCE THEIR YOUNG ONES. Because the young ones are produced in water, they breathe in air with gills. However, the gills change into lungs when the young amphibians develop into adults. This change in form is termed metamorphosis /metamorfisis/. Therefore, hippos, crocodiles, snakes and tortoises whose young never undergo any metamorphosis are not amphibians.
Adult amphibians then move out of water to live on land. The skin of the adult is porous; therefore, adult amphibians live in cool and moist places on land. In water, the mature amphibians breathe in air through their porous skin because they lack gills. Another feature that distinguishes amphibians from reptiles is that amphibians lack scales on their bodies.
Ultimately, we notice that the natural system of classifying living things is the best because it uniquely identifies every living thing based on phylogenetic traits (features organisms have in common because they descended from the same ancestor). Every organism belongs to a distinct ?group? within each of the taxa in the hierarchy of biological classification. Any organism that breastfeed its young is a mammal; thus, a hippo is an aquatic animal but it is a ?true? mammal. A crocodile lives both in water and on land, but it is a reptile. Any animal (eg, crocodile, hippo, frog and capybara) that can live in water and on land may be described as being AMPHIBIOUS, but only frogs, toads, caecilians, newts and salamanders are AMPHIBIANS. The term amphibian is typically used as a noun.
Before leaving, we note the following. The term species /spiisis/ is both singular and plural just as sheep, salmon and offspring are. Unfortunately, many of our science textbooks do contain offsprings. Avoid! The term ?cattle? is a plural tantum (always plural); ?cattle can neither be preceded by the indefinite article ?a? nor be followed by a singular verb.
WAEC is always fumbling here. For example, in integrated science WASSCE November 2014, the stem of No. 10 reads: one difference between a beef cattle and a dairy cattle is that a beef cattle?.
The exams council should have removed the article ?a?. Inserting a needless indefinite article, WAEC was also misled to follow ?cattle? with singular verbs for the options of the same question (has and is). The modifiers/adjectives ?dairy? or ?beef? play no role when grammatical number is concerned; thus, both the expressions ?a cattle? and ?a dairy cattle? are incorrect. To specify a number of cattle, you may use the partitive ?a head of?, eg, my dad has 50 head of cattle (50 bulls or cows or both bulls and cows).
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