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SYMMETRY, LOCOMOTION, AND THE EVOLUTION OF AN ANTERIOR END: A LESSON FROM SEA URCHINS
Author(s) -
Grabowsky Gail L.
Publication year - 1994
Publication title -
evolution
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.84
H-Index - 199
eISSN - 1558-5646
pISSN - 0014-3820
DOI - 10.1111/j.1558-5646.1994.tb05300.x
Subject(s) - biology , asterias , anatomy , obligate , paleontology , symmetry (geometry) , zoology , geometry , mathematics , starfish
Bilaterally symmetrical, “regular” sea urchins in the Family Echinometridae (Class Echinoidea; Phylum Echinodermata) were found to lack a locomotor anterior. Heterocentrotus mammillatus and Echinometra mathaei were observed while locomoting. Members of both ellipsoidal species were found to proceed with their short or long axis foremost with statistically equivalent frequencies. This finding demonstrates that the evolution of bilateral symmetry is not always accompanied by the evolution of a locomotor “anterior” end. The elliptical echinometrid sea urchins provide a particularly appropriate study group for investigating the relationship between the evolution of body form and locomotor behavior. Although the radially symmetrical regular sea urchins, from which the echinometrids sprang, lack a locomotor anterior, all “irregular” echinoids, which are also derived from a regular ancestor but are bilaterally symmetrical, possess an “obligate” locomotor anterior. The symmetry and behavior exhibited by the elliptical echinometrid sea urchins therefore demonstrates that the first irregular echinoids (which exhibit bilateral symmetry by definition) need not have possessed a locomotor anterior as they do today.