All the most interesting of the marine worms belong to the Annelida or Chætopoda, popularly known as the Bristle-footed worms, because their locomotion is aided more or less by the presence of stiff bristles that project beyond the surface of the skin. These are all highly organised worms, mostly with very elongated bodies that are distinctly segmented exteriorly by a number of transverse grooves, while the interior is correspondingly divided into a number of compartments by means of a series of septa.
In addition to the bristles already mentioned, there are often numerous appendages, but these must be distinguished from the more perfect appendages of the arthropods, to be hereafter described; for while the latter are distinctly jointed to the body, and are themselves made up of parts that are jointed together, the former are mere outgrowths of the body-wall. The digestive and circulatory systems are well developed, as is also the system of water tubes that connect the body-cavity with the exterior, while the body-cavity itself is full of fluid.
This group of worms is subdivided into two divisions, the many bristled (Polychæta) and the sparsely bristled (Oligochæta) worms. The latter contain the common earthworms and some less known species, while the former include a number of interesting and even beautiful worms, all of which are marine, and many of them among the commonest objects of the sea shore.
These Polychætes exhibit a great variety of habit as well as of appearance. Some live in crevices of the rocks or under stones and weeds, or make burrows in the sand or mud of the bed of the sea, and roam about freely at times in search of food. They are continually coming within the ken of the sea-side collector, being revealed by almost every overturned stone near the low-water mark, and are often seen crawling over the wet rocks just left uncovered by the receding tide; while their burrows are often so numerous that hundreds may be counted in every few square feet. But many are sedentary species, and these are not so generally known to young sea-side naturalists, who frequently observe, and even preserve, the interesting homes they construct, while less attention is given to the architects that build them.
It is very interesting to observe some of the general differences between the roving and the sedentary species—differences which illustrate the principle of adaptation of structure to habit. The roving species are provided with a lobe that overhangs the mouth, bearing feelers and eyes, and are thus enabled to seek out any desired path and to search for their food. They are provided with bristles and other appendages by means of which they can travel freely over the surfaces of solid objects, and are able to swim well either by undulations of the body, or by fringed appendages, or both. The carnivorous species, too, are provided with strong, horny jaws, and sharp, curved teeth, by means of which they can capture and hold their prey. The sedentary species, on the other hand, unable to move about in search of food, are supplied with a number of appendages by means of which they can set up water currents towards their mouths, and which also serve the purpose of special breathing organs, and, having no means of pursuing and devouring animals of any size, they do not possess the horny jaws and curved teeth so common in the rovers. Their eyes, too, are less perfectly developed, and the tactile proboscis of their free-moving relatives is absent.
Fig. 117.—Arenicola piscatorum
Of the roving worms, perhaps, the Lugworm or Sandworm (Arenicola piscatorum) is the best known. Its burrows may be seen on almost every low sandy or muddy shore, and, being so highly valued as a bait, its general appearance is well known to all professional and amateur sea fishers. It reaches a length of eight inches or more, and varies in colour according to the sand or mud in which it lives. The segments of this worm are very different in structure in different parts of the body. Those in the front of the body have a few tufts of bristles arranged in pairs, while the middle portion of the body has large brush-like tufts of filamentous gills placed rather close together; and the hindmost part has no bristles or appendages of any kind, and is so well filled with the sand or mud that it is quite hard and firm to the touch. As is the case with our common earthworms, the sand or mud is swallowed in enormous quantities, and this is not only the means by which the lugworm derives its food, but also assists it considerably in making its burrows; the extent to which this creature carries on its work of excavation may be estimated by the thousands of little contorted, worm-like heaps of sand that lie on the surface at every period of low water. These little heaps are known as ‘castings,’ and consist of the sand that passed through the worm’s body as the burrowing proceeded.
The Ragworm is another species that is highly valued as bait. It burrows into the odorous mud that is so commonly deposited in harbours and the mouths of sluggish rivers. In this species the segments are similar throughout the length of the body, and the numerous flattened appendages give it the ragged appearance that has suggested its popular name. Quite a number of marine worms closely allied to the common ragworm, and resembling it in general form, are to be found on our shores. Many of these may be seen by turning over stones that are left exposed at low tide, while others hide themselves in snug little crevices of the rock, or in the empty shells of the acorn barnacle and various molluscs; and some species, including one of a bright-green colour, creep freely over the wet rocks in search of food or home, often exposing themselves to the rays of a fierce summer sun.