Allied to the turbellarians are the Spoon Worms or Squirt Worms, some species of which inhabit deep water round our shores, where they burrow into the sand or mud of the bed of the sea. These form the class _Gephyrea_, and consist of creatures with sac-like or cylindrical and elongated bodies, and a protrusible proboscis, which is often of great length. Their bodies are not distinctly segmented, nor do they bear any appendages. The skin is tough and horny, and the body-wall, which is very thick and muscular, is often contracted when the animal is disturbed, thus causing a jet of water to be forcibly ejected. 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.