THE EVOLUTION OF A RACING SNAIL

Modern cephalopods evolved from slow-moving low metabolic rate shelled ancestors. Rapid efficient jet propulsion depends on a large ejectabl e mass, impossible while the mantle cavity was enclosed in a shell. Th e shell was progressively reduced, became internal and finally lost th e capacity to provide buoyancy. The streamlined non-buoyant squid were fast but costs of transport soared. Inevitably, there were changes to the fuel supply system ranging from increases in the gills' diffusion capacity and the cardiac output through to modifications speeding up digestive processes. The loss of the external shell was associated wit h increased sophistication of the brain and sense organs. The great co st of rapid let propulsion is associated with semelparous reproductive strategies following very rapid growth. Many squid have developed fin s as an alternative and more economical, if slower, locomotor system. Some have redeveloped neutral buoyancy, notably by retaining ammonium chloride and reducing muscle mass, a return to life in the slow lane.