We now pass on to the largest and last order of gasteropods--the _Prosobranchiata_--so called because the gills are situated in front of the heart. This group is an important one to the sea-side naturalist, since it contains nearly all the univalve molluscs that are common between the tide-marks of our shores, as well as some abundant species that are protected by a shell of several distinct parts. In nearly all of them the abdomen is well developed, and the shell is sufficiently large to cover the whole animal when the latter is retracted; and the gills, which are either pectinated (comb-shaped) or plumed, are lodged in the chamber formed over the head of the animal by the mantle. The order is often divided into two sections--the _Holostomata_ or Sea Snails, in which the margin of the aperture of the shell is entire, and the _Siphonostomata_, in which the margin of the mantle is prolonged into a siphon by which water passes into the gill chamber. This division does not seem to be very satisfactory, as the sections are not separated by very prominent natural characteristics, but it becomes convenient on account of the great extent of the order. In the _Holostomata_ the shell is either spiral, conical, tubular, or composed of several valves, and the spiral forms are usually closed by a horny or shelly operculum of the spiral kind. The head is provided with a proboscis that is generally non-retractile, and the gills usually extend obliquely across the back, or are attached to the right side behind the head. We shall first consider the lower forms, starting with the family _Chitonidæ_, the animals of which, as the name implies, are covered with a shell that resembles a coat of mail. Some of these creatures are very common on our rocky coasts, and yet their nature is such that they are liable to be overlooked by those who are not acquainted with their appearance and habits. The shell is oval or oblong, often so coloured as to closely resemble the rocks and stones over which they crawl; and the animal is so inactive when left exposed by the receding tide, and its flat under surface so closely applied to that on which it rests, that it looks merely like a little convexity of the rock. But after a few have been discovered the eye becomes accustomed to their appearance, and large numbers may be obtained in a short space of time. The shell will be seen to consist of eight transverse, curved plates, overlapping each other at their edges, and all enclosed in a leathery mantle, which also forms a projecting margin all round. The middle six plates are different from the first and last in that they are grooved in such a manner that each one displays a dorsal and two lateral areas.