Moving onto land has been an evolutionary challenge for many taxa. For the Annel

Question

Moving onto land has been an evolutionary challenge for many taxa. For the Annelida and Mollusca, which Classes (or subclasses) have successfully moved onto land, and what specializations in anatomy, morphology, physiology and life history do we see in these groups that is associated with life on land? For the Arthropod, which major taxa have successfully moved onto land, and what specializations in anatomy, morphology, physiology and life history do we see in these groups that is associated with life on land?

Explanation / Answer

An ever-increasing number of arthropod gene sequences appear to have answered some long-standing questions about the evolutionary relationships of the various arthropod groups. A recent study (Regier, J. C., et al., Nature, 463:1079, 25 February 2010) examined 63 nuclear genes from 75 species of arthropods and concluded that * the crustacea are paraphyletic; that is, the single common ancestor from which all the animals we call crustaceans are descended was also the ancestor of another group, the insects (Hexapoda). So insects are terrestrial crustaceans! * All these groups plus the millipedes and centipedes (Myriapoda) make up a clade designated Mandibulata. * So millipedes and centipedes are more closely related to the crustaceans than to, as once thought, the Chelicerata. Chordates (Phylum Chordata) During their embryonic development, all chordates pass through a stage called the pharyngula [View] with these features: * a dorsal, tubular nerve cord ("1") running from anterior to posterior. At its anterior end, it becomes enlarged to form the brain. * a flexible, rodlike notochord ("2") that runs dorsal to the digestive tract and provides internal support. In vertebrate chordates, it is replaced by a vertebral column or backbone long before maturity. * pairs of gill pouches. These lateral outpocketings of the pharynx are matched on the exterior by paired grooves. In aquatic chordates, one or more pairs of gill pouches break through to the exterior grooves, forming gill slits ("3"). These provide an exit for water taken in through the mouth and passed over the gills. * a tail that extends behind the anus

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