In the earthworms, on the other hand, this coat is thick and composed of many layers.
Various species among those that are predaceous attack smaller insects, hunt in packs crustaceans larger than themselves, insert their narrow heads into snail-shells to pick out and devour the occupants, or pursue slugs and earthworms underground.
Many of our native species spend the day lurking beneath stones, and sally forth at night in pursuit of their prey, which consistsof small insects, earthworms and snails.
They are disposed in two groups on either side, corresponding in the Polychaeta to the parapodia; the two bundles are commonly reduced among the earthworms to two pairs of setae or even to a single seta.
In the earthworms, on the other hand, the epidermis becomes specialized into several layers of cells, all of which are glandular.
The prevalent number of testes is one pair in the aquatic genera and two pairs in earthworms. But there are exceptions; thus a species of Lamprodrilus has four pairs of testes.
Among the aquatic Oligochaeta and many earthworms (the families Lumbricidae, Geoscolicidae and a few other genera) the spermathecae are simple structures, as has been described.
12 and 13 are shown the spermathecae of the genera Hyperiodrilus and Heliodrilus, which are simple sacs ending blindly as in other earthworms, but of which there is only one median opening in the thirteenth segment or in the eleventh.
- Earthworms, rarely aquatic in habit.
Earthworms are divided into the following families, viz.
These ducts therefore have not their exact counterparts in the Oligochaeta, unless we are to assume that they collectively are represented by the seminal vesicles of earthworms and the vasa deferentia.
There are few earthworms or snails.
Again, the well-known action of earthworms may be said to be a biological work; but the resulting aeration of the soil causes edaphic differences; and earthworms are absent from certain soils, such as peat.
In that country, also, the earthworms of Europe are noticed to replace native forms as the ground is broken.
The word was introduced to English readers in a translation (1601) of Pliny's Natural History by Philemon Holland, who defined "insects" as "little vermine or smal creatures which have (as it were) a cut or division betwene their heads and bodies, as pismires, flies, grashoppers, under which are comprehended earthworms, caterpilers, &c."
Aldrovandi in De animalibus insectis (1602) almost contemporaneously distinguished between "terrestrial insects," including woodlice, earthworms and slugs, and "aquatic insects," comprising annelids and starfishes.
All reptiles belong to the class of insects, for the same reasons that earthworms belong to it."
The general practice for many years past among naturalists has been to restrict the terms "Insecta" and "insect" to the class of Arthropods with three pairs of legs in the adult condition: bees, flies, moths, bugs, grasshoppers, springtails are "insects," but not spiders, centipedes nor crabs, far less earthworms, and still less slugs, starfishes or coral polyps.
As pairs in each ring-like segment or somite of the body, and some of these are in all cases retained as gonoducts and often as renal excretory organs (green glands, coxal glands of Arachnida, not crural glands, which are epidermal in origin); but true nephridia, genetically identical with the nephridia of earthworms, do not occur (on the subject of coelom, coelomoducts and nephridia, see the introductory chapter of part ii.
The earthworms of England belong entirely to the three genera Lumbricus, Allolobophora and Allurus, which are further subdivided by some systematists; and these genera form the prevalent earthworm fauna of the Palaearctic region and are also very numerous in the Nearctic region.
It is a remarkable fact that these genera, comprizing a separate family Lumbricidae, when introduced into tropical and other countries, thrive abundantly and oust the indigenous forms. In gatherings of earthworms from various extra-European countries it is always found that if the collections have been made in cultivated ground and near the coast the worms are of European species; farther inland the native forms are met with.
Most earthworms live in the soil, which they devour as they burrow through it.
White not only notes the homes and ways, the times and seasons, of plants and animals - comparing, for instance, the different ways in which the squirrel, the fieldmouse and the nuthatch eat their hazel-nuts - or watches the migrations of birds, which were then only beginning to be properly recorded or understood, but he knows more than any other observer until Charles Darwin about the habits and the usefulness of the earthworms, and is certain that plants distil dew and do not merely condense it.
It might be inferred, therefore, and the inference is proved by facts, that truly oceanic islands have no indigenous fauna of earthworms, but are inhabited by forms which are identical with those of neighbouring continents, and doubtless, therefore, accidentally introduced.
Like the leeches the earthworms produce cocoons which are a product of the glandular epithelium of the clitellum.
So far as is known, the production of cocoons is universal among earthworms and the remaining Oligochaeta of aquatic habit.
The young leave the cocoon as fully formed earthworms in which, however, the genitalia are not fully developed.
The work of earthworms in aiding in the production of the subsoil and in levelling the surface was first studied by C. Darwin, and has since been investigated by others.