Certain fishes, the eel is one, the mackerel another, are said to be scaleless. As a fact their scales are very thin and transparent and so arranged that they are less evident than those of other fish. By taking a little of the skin of one of these fish we can easily detach a few scales for examination. Those of the eel we shall find are very thin and delicate and quite transparent. These and all other fish-scales may be made into permanent slides by mounting in Canada Balsam, as described in our last chapter. The carp, whiting, salmon, sprat, herring and many other fish have scales called cycloid or circular; the term is rather a misnomer because they are not truly circular, but the name is used to distinguish them from other scales. The structure is easily made out with a moderate magnification. Many of these scales, however, exhibit portions more dense than the rest; these dense spots are caused by little particles of lime which may be seen under a higher magnification. [Illustration: 1. A THORN INSECT.--A striking example of protective resemblance. When resting on a thorny twig this little insect is safe from all its enemies. 2. THE HEAD OF PALM WEEVIL.--The long snout distinguishes the weavils from all other beetles. Its very long front legs are also worthy of notice. 3. A LEAF INSECT.--Green in colour, this insect bears a remarkable similarity to a leaf. Its sluggish habits heighten the illusion. 4. THE HEAD OF STICK INSECT.--There are few more curious insects than the stick insects. The specimen illustrated has a very bird-like head. ] Perch, pike, sole and some other fish have much more peculiar scales, known as ctenoid or combed for the reason that their unattached, that is to say their hinder margins, are toothed like a comb. The scales of sharks, dog-fish and rays are called placoid for they are toothed; not only so but their arrangement is frequently quite dissimilar to the scales of ordinary fish. Taking the herring as our example, but a salmon or any other fish would serve equally well, and examining the arrangement of its scales with the help of our pocket lens, we shall find that the scales are fixed to the fish by their forward edges and that each scale partly overlaps its neighbour, as do tiles on a roof. In the shark family, however, the scales are often relatively wide apart, they do not overlap but are imbedded separately in the skin. The scales of rays have each a hard spine projecting from the centre, those of sharks and dog-fish have teeth, and they are teeth not only in appearance but also in structure. The ganoid scales of sturgeon we are hardly likely to meet with. Sometimes these fish are on sale in London and other large towns and a specimen of their scales may be procured. They are bony in structure and, though interesting, require a considerable amount of preparation to render them sufficiently transparent to be examined under the microscope.