Jet propulsion and the giant fibre response of Loligo.

Abstract

IN modern cephalopods the potentialities of jet propulsion have developed to a high degree, and this mode of locomotion has become characteristic of the group. Water is taken into a compression chamber of the body, the mantle, through a wide inlet, the mantle aperture, and squeezed out through a narrow funnel that can point the jet in almost any direction. In such powerful swimmers as the cuttlefish (Sepia) and the common squid (Loligo) the mantle musculature comprises about one-third of the weight of the whole body. Moreover, in decapods possessing the giant fibre system, mantle, head retractors and funnel are provided with a nervous control mechanism especially adapted for ensuring that the compression chamber neither herniates nor leaks and is emptied of all its water in a single compression stroke by simultaneous and maximal contraction of all muscle fibre elements which are innervated1–3. The muscle elements respond to giant fibre stimulation in an all-or-nothing manner (there is no summation or facilitation), and the central nerve connexions of the system are organized so that in life the giant fibres on the two sides of the body act as one unit if they act at all. Thus the giant fibre response is suitable for comparisons of top swimming performances in different animals.