Studies on the Systematics and Anatomy of New Zealand Earthworms
The study of New Zealand earthworms has been extensive, but has been confined principally to the systematics of the group. Only one family of the Oligochaeta, the Megascolecidae, is represented in the endemic fauna, but within this family, over eighty species, belonging to seventeen genera, have been recorded and described. Apart from the Megascolecidae, certain species, lumbricids, worldwide in their distribution, are present and are regarded as having been introduced through the agency of man. The family Megascolecidae is confined almost entirely to the Southern hemisphere, and the southern regions of the Northern hemisphere, and within these regions, the greatest number of species occur in New Zealand, South America, South Africa, and Australia. When the distribution of the Megascolecidae became known, in the late nineteenth century, its sporadic nature evoked a great deal of interest among zoo-geographers, since earthworms, being terrestrial, and unable to tolerate immersion in salt water, form an ideal basis for the consideration of dispersal problems among terrestrial animals as a whole. The interest thus aroused in the Megascolecidae led to much work on the group in New Zealand. Michaelsen (1913 (b)) accounts for the predominance of the Megascolecidae in the southern continental areas by postulating that originally the family had a wide distribution in the nothern and southern continents, and that other families (e.g. the Glossoscolecidae), evolved more recently in the northern continents, have gradually superseded the Megascolecidae in all but the most remote regions of their original area of distribution. Matthew (1915) came to a similar conclusion in regard to the origin of present southern faunas in the course of his work on the distribution and evolution of the Mammalia. Evidence in favour of Michaelsen's conclusions can also be derived from the distribution of slugs, spiders, Collembola, Coleoptera, littoral Echinoderms, Polychaeta and Brachiopoda in the southern land masses.