The following table, taken from Darwin's book (1881/1985, p. 90), summarises his findings on the manner in which worms draw various kinds of objects into the mouths of their burrows:

Nature of object	Drawn into the burrows, by or near the apex	Drawn in, by or near the middle	Drawn in, by or near the base
Leaves of various kinds	80	11	9
of the Lime, basal margin of blade broad, apex acuminated	79	17	4
- of a Laburnum, basal part of blade as narrow as, or sometimes a little narrower than the apical part	63	10	27
- of the Rhododendron, basal part of blade often narrower than the apical part	34	..	66
- of Pine-trees, consisting of two needles arising from a common base	..	..	100
Petioles of a Clematis, somewhat pointed at the apex, and blunt at the base	76	..	24
- of the Ash, the thick basal end often drawn in to serve as food	48.5	..	51.5
- of Robinia, extremely thin, especially towards the apex, so as to be ill-fitted for plugging up the burrows	44	..	56
Triangles of paper, of the two sizes	62	15	23
- of the broad ones alone	59	25	16
- of the narrow ones alone	65	14	21

Darwin also studied the interiors of earthworms' burrows. He noted that worms were driven by their instincts to create basket-like structures, held together with leaves and worm castings, to protect the openings of their burrows. He became convinced that the skill with which they constructed their burrows also required some intelligence, noting that earthworms pressed pointed pine needles into the sides of the interior of the "basket" in a way that minimised injury to their bodies by the leaves, when entering or exiting their burrows. Darwin saw this as particularly remarkable, as the pine needles did not come from a tree that was native to the area. This suggested to him that the behaviour was not innate.