In brackish water of various densities many Polychaetes live; Arenicola especially is regardless of the character of the medium, and Nereis diversicolor appears to withstand considerable admixture of fresh water.
The majority of the Polychaetes occur "inshore," that is, between tide-marks and in shallow water down to 20 fathoms; but they occur at all depths more or less abundantly, and some have been dredged from depths of more than 3000 fathoms.
The nature of the soil composing the shore has a good deal to do with the number of worms to be found there; thus in calcareous districts they are fewer than in places where harder rocks, such as granite, form the shore line, for the chalk or limestone wears away more quickly, and exposes to destruction the worms which may have sheltered in its crevices: further, it does not give so permanent a place of attachment to seaweeds, on which many Polychaetes feed. The calcareous rocks, too, are more likely to be traversed by springs of fresh water, which is not to the taste of the worms. The sand resulting from the destruction of the rocks, whether hard or soft, is of itself unsuitable to the majority of worms, which are most abundant where mud containing decaying vegetable matter is mixed with the sand: this, which gives a firmer consistency to the soil, so that the burrows retain their form better, supplies food for the burrowers.
General Habits.—The division of the Polychaetes into the "Errantia" or free-swimming and wandering forms, and "Sedentaria" or tubicolous and sedentary forms, is a misleading mode of classification, for as a matter of fact only a comparatively few forms are really free-swimming throughout life; the majority, even if they do not form definite tubes, burrow galleries for themselves in the soil, and these burrows are in many cases only rarely left; this is true of both groups. Amongst the "errant" Polychaetes nearly all the Eunicidae secrete a parchment-like tube, and some Polynoids form mud tubes. Among the "sedentary worms" there are forms which merely burrow; while Myxicola readily leaves its gelatinous tube and swims freely; Pectinaria carries its house with it as it moves about, and Polycirrus, a Terebellid, does not form any tube at all.
Owing to their sedentary habits, quite a representative collection of genera may be made, especially at a spring tide, at any seaside place which is provided with a sandy shore, and with rocks and seaweed. The larger species, however, require to be dredged, and the best time is at night, for then many forms which during the day are concealed in their burrows, will be issuing forth to obtain food.
It may be useful to give instances of worms occurring in various situations between tide-marks. Throughout pretty well the whole of the area left uncovered by the tide, even up to nearly high-water mark in many parts of the coast, the cylindrical "castings" of sand and mud, forming little heaps, indicate the burrows of Arenicola, the common "lug-worm"; these "castings" have passed through the worm's body, having been swallowed during the process of burrowing as well as for the purpose of obtaining food, as in the case of the earthworms. Rather nearer the water may be seen little tufts of sand-threads, about an inch high, springing from a short piece of cylindrical, sandy tube rising up out of the sand; this is the head end of the tube of Terebella conchilega (Fig. 153).
Amongst the rocks may be found loose stones of different sizes; on lifting them up, various kinds of worms may be brought to light, according to the locality, the time of year, the position with respect to the sea, and so on. Polynoë is pretty sure to be present somewhere near low-tide mark; the number of species is considerable, and their colouring very varied: but as the worms have a habit of remaining still on the under surface of the uplifted stone, the observer may easily overlook them.
Other worms occur below the stones, more or less buried in the sand or mud; for instance, a small Nereis may be lying in its temporary burrow immediately underneath, and will at once withdraw from the now injured part of the burrow; while deeper in the mud or sand, especially in rather highly-smelling mud, little red worms are abundant, such as Scoloplos, Nerine, Capitella, and others. By digging near low water one may find Nephthys, Glycera, and others burrowing or hiding in the soil.
In rock pools, or sandy stretches amongst rocks kept moist and cool by abundant Fucus, one may see under stones the red or yellow gill filaments of Cirratulus and of Terebellids protruding from their burrows and tubes, while other worms are to be met with in clefts of the rocks, and amongst the roots of Laminaria.
Still farther out, below low-water mark, where one must wade, can be seen the beautiful branchial crowns of various Sabellids protruding from their tubes; but care is necessary on approaching these worms, as eyes are, in many cases, present on the branchiae and a shadow is readily perceived; then the brightly-coloured tuft disappears, and only a piece of sandy or muddy cylindrical tubing remains to tell where the Sabella has withdrawn. In order to obtain the worms one must dig quickly and deeply before they have been disturbed; for the tube is of considerable length, and the inhabitant withdraws to the bottom of it. Some of these soft-skinned worms have the power of boring into hard rocks,[[343]] though by what means they do so is uncertain.[[344]] Polydora ciliata makes a tube of mud projecting from the mouth of U-shaped galleries in chalk, limestone, shells, and even shale; it has no hard jaws or other structures sufficient to account for the holes, but it is possible that the specially strong chaetae on the sixth segment may be of some use in this work. Other lithodomous worms are Sabella saxicava and Dodecaceria concharum, which is a common little borer, forming galleries in oyster-shells, etc.