Reasons for Uneven Distribution of Power
The causes for uneven distribution of the power of regeneration among animals are not noticeably known. From a survey of the regeneration in these types of groups it appears that lower or simpler organized types of animal life have greater regenerative power than the much more complex, higher evolved forms. That is, they are capable to restore normal structure from small parts of the original individual that usually is not possible for more highly in animals.
Though, this cannot be regarded as a rule. Many phylogenetically lower animals with simple organization like nematodes and related forms are unable to regenerate at all, whereas more highly evolved forms such as echinoderms and many annelids have substantial powers of regeneration of lost parts. Within similar phylogenetic group, the urochordates, the colonial species (Clavelina, Perophora) are good at regeneration while this ability is absent in the solitary forms such as Ciona and Herdmania. Similarly, within amphibia, urodeles can regenerate several structures throughout life but most anurans can regenerate limbs (and also tails) during larval period only. Likewise, adult insects cannot regenerate their leg though nymphs or larvae can.