Hermit crab shells instead, as shown by a specimen of "Palaeopagurus vandenengeli" from the Speeton Clay, Yorkshire, UK from the Lower Cretaceous. Hermit crab Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the approximately 1,110 species possess an asymmetrical abdomen that is concealed in a scavenged mollusc shell carried around by the hermit crab. Most species have long, spirally curved abdomens, which are soft, unlike the hard, calcified abdomens seen in related crustaceans. The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract. Most frequently,
On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection the connection between natural selection and systems theory. On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection is the title of a joint presentation of two scientific papers to the Linnean Society of London on 1 July 1858: "On The Tendency of Varieties to Depart Indefinitely from the Original Type" by Alfred Russel Wallace and an "Extract from an unpublished Work on Species" from Charles Darwin's "Essay"