. All mammals without any exception are characterised by :
(a) viviparity and biconcave red blood cells
(b) extra-abdominal testes and a four chambered heart
(c) heterodont teeth and 12 pairs of cranial nerves
(d) a muscular diaphragm and milk producing glands
Solution: Mammalian teeth became differentiated to perform specialized functions such as cutting seizing, gnawing, tearing, grinding or chewing. Teeth differentiated in this manner are called heterodont.
Cranial nerves conduct sensory information from specialized receptors to the brain. Mammals contain12 pairs of cranial nerves.
Monotremes are egg laying animals. Members of sub-class Prototheria contain abdominal testes.