CONTRACTILE CONNECTIVE-TISSUE IN CRINOIDS

Active movements in animals are usually attributed to cellular protein engines, e.g., the actin-myosin system of muscle cells. Here we repor t the first evidence of an extracellular contractile connective tissue , which we have found in sea lilies and feather stars (Echinodermata, Crinoida). These marine animals have arm muscles that ave antagonized not by other muscles, but by ligaments consisting of extracellular fib rils interspersed with neuron-like cell processes. Contractile cells a re lacking, yet these arm ligaments actively contracted upon stimulati on. The ligaments stayed in a contracted condition even after the stim ulus had stopped. The stresses generated were lower than those of typi cal skeletal muscles. Additional data from crinoid cirri, which lack m uscles entirely, corroborate the hypothesis that the connective tissue of the ligaments is contractile.