Annulosa, St Thomas calls them: they are now known as Annelidae, worms, centipedes, and the like. The cutting of an Annelid in two is not a case of reproduction. But in the lowest animal life, that of Amoebae, there is a true reproduction by 'fissure'; as also in the propagation of plants by cuttings.
The kindness of a medical friend suplies me with the following statement:
"When the body of an Annelid, say an earthworm, is divided, as by the stroke of a spade, the animal does not necessarily die, does not necessarily live. The principal nerve gangia are situated in the head, and though the severed part, remote from this, so-called, central nervous system, will have no restorative power and will die, the segment containing the nerve masses -- 'brain' -- may restore or reproduce the missing opposite extremity, or a semblance of it. But, if the injury were very near the head, so that almost all the vital organs, viscera, etc., were included in the segment remote from the nerve ganglia, death would occur, not even the nerve ganglia in the head having the power to restore or reproduce an almost entirely new body, viscera, blood vessels, etc. So, while one part may live, both parts may die. It depends upon the amount and importance of the part or parts to be reproduced, or restored."
Of God and His Creatures: 2.86