The Dog Periwinkle (_Purpura lapillus_) abounds on all our coasts and is remarkable for the production of a dull crimson or purple fluid that may be obtained from it by pressing on the operculum. This fluid turns to a brighter colour on exposure to air, and is said to have been used largely in former times as a dye. It will be seen from our figure that the spire of this shell is shorter in proportion than that of _Buccinum_; but both are alike in that the operculum is made up of layers with a nucleus on the external edge. The other species figured is _Nassa reticulata_, popularly known as the Dog Whelk, and characterised by a tooth-like projection of the inner lip close to the anterior canal. It is very common near low-water mark, where it may be seen crawling over the rocks on its broad foot, from which project two hornlike appendages in front and two narrow tails behind. [Illustration: FIG. 182.--_Murex erinaceus_] From the last family of the gasteropods (the _Muricidæ_) we select two common species--_Murex erinaceus_ and _Fusus antiquus_ (Plate V.). In both these the anterior canal of the shell is straight and the posterior wanting. The eyes are on the tentacles, and there are two plumed gills. Both are carnivorous species, feeding on other molluscs; and the former is said to bore through the shells of its prey with the prominent beak of its shell. _Murex_ may be readily distinguished by the prominent longitudinal ridges of the thick shell, its rounded aperture, and by the partly closed canal running through the beak. It is known to fishermen as the Sting Winkle; the other species is called the Red Whelk in some parts, and in Scotland is known as the Buckie. Like the common whelk, it is dredged largely for the market, and is said to be far more esteemed than the former, from which it may be distinguished by the fusiform shape of the shell and the long straight canal. We now pass to the last and highest class of the mollusca, called the _Cephalopoda_ because they have a number of arms attached to the head, round the mouth. Unlike the majority of molluscs they are bilaterally symmetrical: and are much more highly organised, in some respects even making an approach to the vertebrates. Thus they generally have an internal hard structure, either horny or calcareous in structure, representing the vertebral column, and the circulatory system consists of arteries and veins, connected by minute capillaries. The corpuscles of the blood are also similar in form to those of the vertebrates. Externally they are all naked, with the exception of the nautilus and argonaut of the warmer seas.