They furnish very interesting illustrations of protective colouring, the upper surface always closely resembling the ground on which they rest and feed; and thus they are not only protected from their own enemies, but are enabled to lie unseen by the animals that form their prey. Those which live on sandy shores are finely spotted with colours that closely imitate the sand, while those that lie on mud are of dark and dingy hues. Others, again, are irregularly marked with spots of various sizes and colours that resemble a gravelly bottom; and most species are still further protected by their habit of throwing sand or mud on the top of their bodies by means of their dorsal and ventral fins. Small flat-fishes, especially young Plaice and Flounders, live so close to the shore that they are often left behind in rock pools and sandy hollows by the receding tide, and it is very interesting to observe the habits of these in their natural conditions. It will generally be noticed that it is most difficult to detect them while they are at rest; and when disturbed, they usually swim but a short distance, settling down very abruptly, and immediately throwing a little sand over their bodies by a few vibrations of their fins. Another peculiarity of some of the flat-fishes is their indifference to the nature of the water in which they live. Flounders may not only be caught in the estuaries of our rivers, but they even ascend to, and apparently live perpetually in, perfectly fresh water. In many instances they may be seen miles from the sea, and even flourishing in little fresh-water streams only a few feet in width. Thus they may be found in numbers in the upper waters of the small rivers of the Isle of Wight and of many streams of the mainland. The principal British flat-fishes are the Plaice (_Pleuronectes platessa_) and Flounder (_P. flexus_) above mentioned, and also the Sole (_Solea vulgaris_), the Lemon Sole (_S. aurantiaca_), the Turbot (_Rhombus maximus_), and the Halibut (_Hippoglossus vulgaris_); and as all these are well-known food-fishes it is hardly necessary to describe them. [Illustration: FIG. 233.--THE LESSER SAND EEL] Sand Eels (family _Ophidiidæ_) resemble the true eels in the general form of their elongated bodies, but may be readily distinguished by their bright silvery colour, the large gill-openings, and the more strongly developed dorsal and ventral fins, the former of which extends almost along the whole length of the back. The lower jaw is also longer than the upper.