Infant tissues, organs, body parts or reflexes in an adult organism
&quot;Remnant&quot; is the closest term I can think of; &quot;residual&quot; is also used. You can find both used to describe the foramen ovale for example, which typically closes at birth (and has served its purpose at that point), but remains patent in a substantial proportion of adults, sometimes causing symptoms.
However, these terms are of course not specific for the circumstance you describe; I don't know of any that is.