Annelids and molluscs

Annelids
It is important to reflect on the fact that, unlike nematodes, annelids are worms with a real coelom and that phylogenetically they occupy a very different position respect to the former. The main representatives of this particular component of soil fauna are the oligochaetes, i.e. common earthworms with a circular cross-section, characterised by very accentuated body metamerism (the repetition of identical anatomical segments along the body’s principal axis) and by a length that often exceeds 10 mm. Respect to the taxonomic groups discussed so far, oligochaetes are animals characterized by a minor dependence on the amount of water present underground. Contrary to  what one might think, annelids, and oligochaetes in particular, are animals that have been studied a lot by biologists in the past, starting fromCharles Darwin who took particular interest in them. In fact, their importance in the ecological equilibrium of the soil is undisputed since they participate in all the mixing processes of organic substances and mineral components present in soil particles, increasing their fertility.
Molluscs
Molluscs, whose dimensions vary from few mm to a few tens of mm, have been neglected for a long time in soil biology studies for the simple fact that historically they have been considered more as generically epigeal animals rather than real hypogeal ones. The latter idea, however, albeit incorrect, had a logical explanation since, in numerical terms, molluscs rarely appear among the animals that dominate life in the soil. Currently, however, recently updated information on the biology and ecology of molluscs indicates that these animals are part of many trophic relations in food webs characteristic of the hypogeal environment. From this viewpoint, the main taxonomic group is that of the Gasteropoda pulmonata (with or without shell) whose ecological function must be prevalently related to the demolition of organic material present in the topmost layers of the soil. Often, however, the specimens of this subclass have herbivorous feeding habits since the main food of many species includes leaves and other decaying vegetation

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