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. [Illustration: 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.