Serendipity: the faculty of making happy and unexpected discoveries by accident Roberts T.J., Azizi E. 2011. Flexible mechanisms: the diverse roles of biological springs in vertebrate movement. J. experimental Biology 214: 353-361.

The commonest device that confers motility in animals is muscle The commonest device that confers motility in animals is muscle. But there are some oddities in how animal movement can be achieved: erection of antennal hairs in mosquitoes occurs by hydration. Some plants achieve motility by hydration, e.g., Mimosa: the imbibition of water --and its an aspect of their growth. The hairs on the antenna of a mosquito start out lying flat against the antennal axis: recumbent. An annular pad of protein near the base of each hair absorbs water and causes the hair to swing normal (erect)to the axis of the antenna Vogel, Life’s Devices p. 256

Pecten, scallop: water-jet swimming propulsion Seawater exits from 2 openings near the hinge as the valves are adducted: this jets the scallop forward.

Water jetting as a means of locomotion: dragonfly nymph (immature): jet out water from last chamber of gut, rectum locomotion combined with irrigation of rectal gills for gas exchange

Muscular hydrostats Definition: a skeletal system of body or body appendage movement that relies upon the incompressibility of muscle itself, i.e., there is no fluid-filled cavity. Muscle is mostly water and it is a tissue that can change its shape and this permits it to achieve changes in body shape by virtue of its relative incompressibility.

Muscular hydrostat Definition: a skeletal system of body or body appendage movement that relies upon the incompressibility of muscle itself, i.e., there is no fluid-filled cavity to translocate forces. So the muscle itself does the translocation. Muscle is mostly water and it is a tissue that can change its shape and this permits it to achieve changes in body shape by virtue of its relative incompressibility. Gosline J.M., & Demont M.E. 1984. Jet-propelled swimming in squids. Scientific American 252: 96-103. http://vimeo.com/5879164 Comment videos online

The seawater within the mantle cavity of the squid is not functioning as a hydrostatic skeleton. But it is the basis of the animal's jet propulsion, which in turn depends upon the incompressibility of seawater. When the radial muscles of the mantle contract, the volume of the mantle cavity is increased and seawater is drawn in. When the circular muscles of the mantle contract, the volume of the mantle cavity is decreased and seawater is squirted out.  The action-force of the jetted seawater creates a reaction force that pushes the squid in the opposite direction: opposite to whatever direction the funnel is pointing. Valves control intake of water into mantle cavity at sides . Pressure build up in seawater inside mantle cavity (circulars contract) forces the inner flaps of the funnel against the mantle wall

Three structures interact: 1) collagen fibres (connective tissue) make a tunic that prevents longitudinal dimension change 2) radial muscles contract to thin the mantle wall and 3) circular muscles of the mantle contract to thicken the wall. Circulars and radials are antagonists. The mantle is a muscular hydrostat and its volume must stay constant (just as if it were a fluid-filled cavity). Because of the collagen ‘tunic’ the mantle cannot get longer in the A to B dimension: but it can change in girth. A B

Escape Jet Cycle of squid relaxed Mantle wall Internal organs relaxed Radial muscles contract to cause: hyperinflation: seawater intake into mantle cavity: outside diameter of mantle increases by approximately 10% over resting diameter (girth increase); cavity volume increases 22% re relaxed volume, wall thins. Circular muscles contract to bring mantle to about 75% of its relaxed diameter, radials restored to precontracted length (girth decrease): volume drops & pressure rises sharply , forcing the inlet valvesl against the mantle wall and leaving only the funnel as exit. Escape Jet Cycle of squid relaxed contracted relaxed

The mantle wall can function as a muscular hydrostat -- a fluid skeleton without a distinct fluid chamber -- that makes antagonists of the radial and circular muscles: contraction of one kind of muscle restores the other to its relaxed state via this type of fluid skeleton. The radial and circular muscles become coupled as antagonists by virtue of their own tissue being significantly water and so incompressible – and because the mantle cannot lengthen. Because the mantle is incompressible it must retain an overall constant volume; and it cannot get longer as mantle muscles contract because of the collagen fibre tunic that prevents any movement in that direction. Thus, it can only increase or decrease in thickness – at the same time changing its overall diameter and the capacity of the mantle cavity. When the radials contract the mantle walls must get thinner and the walls move apart -- to maintain hydrostat volume. Conversely when the circulars contract the mantle wall must get thicker as the overall outside diameter of the mantle decreases. If there were no inextensible fibres, if the animal’s mantle was not in a jacket of fibres preventing it from lengthening, then the radials and the circulars could not have an antagonistic effect on each other.

Echinodermata tube feet/podia Calcareous plates in dermis Ca Carbonate ‘ossicles’ Water vascular system: tubes: ring canal and radial canal, filled with coelomic fluid, amoebocytes Ambulacral groove tube feet, distal extremity is a disc stiffened internally by small ossicles: rosette, epithelium: mucus-secreting cells, disc muscles create partial vacuum, disc acts like a suction cup, adhere to valve of bivalve prey Ampulla: fluid reservoir with ampullar muscles; spiral connective tissue in podium wall enables protraction Lateral canal: tiny one-way valve eliminates backflow when ampullar muscles contract Retractor muscles, postural muscles; podia push animal forward

Taken from Brown, F.A. Selected Invertebrate Types, p. 521

Branchiostoma amphixous