Locomotion--More Molluscs A. Staying in one place--the limpets and other snails 1. Foot is somewhat circular/oval in shape which maximizes surface area. 2. Foot consists of muscle fibers that run both anteriorly/ posteriorly and dorso-ventrally. The latter fibers allow the foot to conform to the surface area of the substrate the animal is situated on. 3. Adherence of foot to substrate is by tension and by the fact that a slight vacuum is produced between the foot and the substrate. This is because the foot fits the substrate 4. Often with snails such as limpets, the animal will show homing ability, that is, it will return to the same location after venturing about. Thus not only does the foot fit to the substrate, but also the edge of the shell. B. Movement of snails 1. Wave of perstaltic contraction moves the length of the foot. 2. As the wave passes, part of the foot is applied to the surface and part is lifted off the surface. This does two things. When the foot is lifted off the surface, it can be moved forward. The part of the foot still attached to the surface retains the connection to the substrate preventing dislodgement of the creature. 3. In some snails, the wave may actually have a right and left component with the movement down each side slightly out of phase to the other. 4. Passage of the wave implies that only part of the foot is on the substrate at any one moment in time. This means that the animal can move over very easily without damage to the foot projections, ridges, etc. in the environment. This also explains why the animal can move over the edge of a razor blade without damage. C. Bouyancy in Cepahlopods 1. Fixed shells--Early cephalopods had cone shaped shells with compartments. These compartments could be filled with gas. Amount of gas in each chamber could be regulated through the sipuncle. Problem with regulation of gas mixture is that if too much is released, the creature will sink too quickly, while if too much is secreted, it will rise too quickly. To withstand pressure, the shell has to be very thick. Strength of shell has also been accomplished by having septa in addition to a thick shell. Spirula has a slightly coiled, but very thick shell, whereas Nautilus has a coiled shell, somewhat thinner, but with septal walls. 2. Fixed shells with mixture of gas and water--Found in cuttlefish. Cuttlebone has plate like structure dorsally that provides strength to the cuttlebone, and a porous, chambered area that contains gas/fluid mixture. X-ray studies of living cuttlefish show that they are able to regulate the gas volume in the cuttlebone to adjust their bouyancy. 3. Use of light ions--Many squid are able to regulate their bouyancy by regulating the nature of the ions they retain in their body fluids. The most important compound is the ammonia that is produced as a by-product of metabolism. These squid have a "pen" (an elongate structure formed of chitin that provides structurally strength to the animal's body) and tissues that are very flacid. Some investigators have described these creatures as "bags of water and ions". D. Locomtion in squid 1. Use mantle muscles that force water out of mantle cavity. Area between head and mantle has a valve which allows water to move into the mantle cavity, but when mantle wall contracts the water can only pass back out via the siphon. The organism aims the siphon and uses jet propulsion to move. 2. Mantle must contract in a coordinated fashion. This is handled by giant nerve fibers of different lengths and diameters. Rate of conduction in "naked" nerve fibers is correlated with diameter of fiber. Thus fibers going to the distal portions of the mantle are not only long, but large in diameter. Those going shorter distances are smaller in diameter. 3. Squid also use lift produced by fins to maintain vertical position. These fins if large, such as those in the cuttlefish, may also produce a propulsive force. This happens if a wave of motion moves down the length of the fin.