Polyisomerism and Anisomerism in Cranial and Dental Evolution among Vertebrates

The researches of Beecher, Walcott and others on the Cambrian brachio- pods show that some of these animals are extraordinarily generalized in appearance, in the sense that the body is composed of a large number of homodynamous segments, each equipped with similar appendages.Prob- ably this condition is not as primitive as it looks and it would not be sur- prising if some day it may be proved that the remote ancestors of the trilobites had only a few segments, perhaps three.But for the purposes of the present paper the many segments of the trilobite Triarthrus may be assumed as the starting-point for the highly differentiated conditions illus- trated in the typical crabs; in these some of the segments have fused to form a cephalo-thorax, while the paired appendages are greatly reduced in number but highly differentiated.We may thus take the conditions in Triarthrus as an example of what I shall here name polyisomerism, the state in which many homologous parts, or polyisomeres, are arranged along any primary or secondary axis, whether straight or curved.The highly differentiated condition in the typical crab, on the other hand, may be named anisomerism, that is, the state in which one or more parts are emphasized at the expense of the rest, while the original number of separate parts is usually reduced, either by fusion or by elimination.Polyisomerism is a more inclusive term than metamerism, which has reference only to homologous parts arranged in an anteroposterior series, whereas in polyisomerism the axis may be anteroposterior, oblique, trans- verse or curved in any direction.Nor is it synonymous with the meristic variation of Bateson, which is concerned with numerical departures from the norm.Polyisomerism obviously results from a process analogous to or identical with budding, while anisomerism results from what Pezard and Huxley call heterogony, or unequal growth of one part of a poly- isomerous series.Polyisomerism corresponds partly to what Cope in 18711 called anteroposterior repetitive acceleration.