The response of marine worms to unilateral tension of their musculature.

Abstract

It has been shown that in Lumbricus 1 and Nereis 2 the anterior segments of the animal respond to passive bending of the posterior part, by active turning of the head in a direction parallel with the new orientation of the tail. This has been termed the homostrophic reflex. The receptors for the reflex are distributed throughout the animal but the efferent nerves are limited in their origin to the anterior 20 segments in Lumbricus, and to the first 2 or 3 segments in Nereis. The afferent impulses travel forward by way of the ventral nerve cord. Bending of the head end does not affect the direction of backward locomotion, hence the mechanism of the homostrophic reflex exhibits polarity. Similar observations 3 have been recorded for Diplopods and for the larvæ of Tenebrio molitor.

A systematic study has shown the homostrophic reflex to play a part in the behavior of a number of nereid worms of the Mediterranean, namely, Eunice gigantea, Eunice siciliensis, Holla parthenospeia, Nephthis scolopendroides, Sthenelais dendrocepis, and Psammolyce arenosa. In these forms the homostrophic reflex disappears with removal of the head ganglia. In the case of very long forms, such as Halla (80 cm.), bending the body posterior to the middle is without effect, but if the bend is made 20 cm. back of the head, the latter promptly orients itself at right angles to the segments immediately behind it.

No evidence of homostrophy was found in the nemertine worm, Cerebratulus marginatus, nor in the flatworm Yungia aurantiaca, nor in the Sipunculida. It thus appears necessary to the occurrence of the homostrophic reflex that the ganglia of the central nervous system be numerous and extensively distributed as in the segmented animals.