Category : 11th Class
Exceptions are inherent in biology due to evolutionary divergence. Not only living organisms, but viruses and biomolecules also exhibit exceptional forms. A student of biology must be prepared to accept and enjoy this gesture of nature.
Some exceptions have been explained logically, while for others, the reasons are yet to be searched out. Following are noteworthy exceptions in zoology,
(1) Mammals are usually terrestrial, fossorial or arboreal but bat is only flying and whale and seals are aquatic mammals.
(2) Mature mammalian RBCs are devoid of nucleus except camel.
(3) The aquatic larval stage of some salamanders like Ambystoma is able to reproduce (paedogenesis) and attain sexual maturity. It is called neoteny. Total neoteny is found in Necturus, Siren and Proteus.
(4) Heart is three chambered in reptiles but four chambered in crocodiles. It is two chambered in fishes but three chambered in lung fishes like Protopterus which possess lung-like structure beside gills.
(5) DNA is double stranded in all cells and DNA viruses but single stranded in the bacteriophage \[\varphi \times 174\] and M-13. Likewise RNA is single stranded in all the cells and RNA viruses but double stranded in reovirus, rice dwarf virus and wound tumour virus.
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