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Summarized from Lee, 1985, and Edwards and Bohlen, 1996.

Summarized from Lee, 1985, and Edwards and Bohlen, 1996.

to more than a meter (e.g., the giant Gippsland earthworm of Australia, Megascolides australis) (Fig. 4.61). Morphological details differ greatly among earthworm groups and many such details (e.g., position of reproductive organs) are used in taxonomic distinctions among species (Dindal, 1990). Nonetheless, a number of features are common to most earthworms (Fig. 4.62). In general, earthworms consist of a simple, tube-within-a-tube body plan, the outer tube constituting the body proper and the internal tube comprising the alimentary canal. Ingested material (e.g., mineral soil, particulate organic matter) is drawn through the mouth into a muscular buccal cavity and then through the pharynx into the esophagus. Many species have a muscular esophageal gizzard that grinds and mixes food material as it passes through. The esophagus in many species also contains a calciferous gland that functions in calcium metabolism and regulation of CO2 levels in the blood. The remainder of the gut consists of the intestine that, in many endoge-ic species, has an infolding of the gut wall known as the typhlosole, which greatly increases the absorptive surface area of the intestine. The overall length of the gut and configuration of the typhlosole vary with species, probably as a function of diet.

Earthworms are hermaphroditic, each individual possessing male and female reproductive organs (testes, ovaries, and associated structures) (Edwards and Bohlen, 1996). During sexual reproduction, sperm is exchanged between two individuals and stored in sperm sacs or sper-mathecae. This sperm is later released, along with eggs, into cocoons secreted by the glandular clitellum, which is the characteristic thickening or saddle-shaped structure often seen around several anterior

FIGURE 4.61. The Australian giant Gippsland earthworm, Megascolides australis, measuring up to 3m in length (from Blakemore, 2002). (Alan L. Yen, with permission.)
FIGURE 4.62. General external characteristics of representatives from three earthworm families. (a) Acanthodrilidae. (b) Megascolecidae. (c) Lumbricidae (from Blakemore, 2002).

segments of sexually mature individuals (Fig. 4.63). One to several embryos may form within each cocoon, depending on earthworm species. Some earthworms reproduce parthenogenetically, whereby an ovum develops without fertilization by sperm. Parthenogenesis provides an effective means by which certain species can establish populations in new habitats; such species often are the successful peregrines and anthropochores discussed previously.

Earthworms are often grouped into functional categories based on their morphology, their behavior and feeding ecology, and their microhabitats within the soil (Lee, 1959, 1985; Bouché, 1977, 1983; Lavelle, 1983). These categories describe the ways by which different earthworm species utilize resources within a soil volume (Table 4.11) (Fig. 4.64). Epigeic and epi-endogeic species are often polyhumic (prefer organically enriched substrates) and utilize plant litter on the soil surface and carbon-rich upper layers of mineral soil; poly-, meso-, and oligohumic endogeic species inhabit mineral soil with high (e.g., the rhizosphere), moderate, and low organic matter content, respectively; and anecic species exploit both the surface litter as a source of food and the mineral soil as a refuge in which they make permanent burrows. The familiar Lumbricus terrestris is an example of an anecic species, constructing burrows and pulling leaf litter down into them. In contrast, Bimastos parvus (the American log worm) exploits leaf litter and decaying logs, with little involvement in the soil—an epigeic species. The ubiquitous

Cocoon forming

Cocoon forming

in lumbricid earthworms (from Edwards and Bohlen, 1996).

European lumbricid, Aporrectodea caliginosa, and several megascole-cids (e.g., Diplocardia spp., native to eastern North America) are endo-geic in life habits. Some earthworm species appear to be intermediate between these categories; for example the epi-endogeic Lumbricus rubellus, which can inhabit litter layers and form shallow horizontal burrows. Even though some species may not exactly fit, these categories

TABLE 4.11. Ecological Categories, Habitat, Feeding, and Morphological Characteristics of Earthworms

Category

Subcategory

Habitat

Food

Size and pigmentation

Epigeic

Epigeic

Litter

Leaf litter,

<10 cm, highly

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