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How do the families Caeciliidae and Ichthyophiidae differ in whether they have tails?

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Zoom in on the serpentine locomotion of the limbless, amphibious caecilian over soil — Gymnophiona Caecilian moving over soil.

Within the order Gymnophiona, the families Caeciliidae and Ichthyophiidae differ significantly regarding the presence of a tail:

Family Caeciliidae: Members of this family do not have a tail.1
Family Ichthyophiidae: Members of this family possess a tail.1

The presence or absence of a tail is one of the key morphological characteristics used to distinguish between the various living families of caecilians.2