Although, as we have already hinted, the body of a mollusc generally bears but little resemblance to that of the typical elongated and segmented worm, yet the study of the earliest stages of the former shows that a certain relationship exists between the two sub-kingdoms, the newly hatched mollusc being often a minute free-swimming creature with expanded lobes fringed with cilia, and bearing a resemblance to certain of the Rotifers, Moss Polyps, and other animals that are included among the _Vermes_. But in the adult molluscs this resemblance is lost, these creatures being generally easily distinguished from all others by certain well-marked external features, as well as by internal characters that are peculiar to them and fairly constant throughout the group. The external shell, where it exists, is usually composed of one or of two parts, and therefore we speak of univalve and bivalve molluscs; and no internal skeleton of any kind is to be found except in the division containing the Cuttlefishes, the ‘bone’ of which is one of the common objects washed up on our shores by the breakers. In all the molluscs there is a well-formed digestive tube, and often a complex arrangement of small teeth which sever the food by a rasp-like action. There is also a well-formed heart, consisting of two or more cavities, by means of which the blood is forced through the body; but, as a rule, blood vessels are either few or absent, the blood being driven through spaces between the tissues that serve the same purpose. [Illustration: FIG. 126.--LARVÆ OF MOLLUSCS _v_, ciliated ‘velum’; _f_, rudimental foot] The nervous system consists of a few masses of nerve substance (_ganglia_), connected by nerve cords, and sending off fibres to various parts of the body, the principal ganglion being one situated close to the mouth, and often surrounding the first portion of the digestive tube. The animals of this sub-kingdom are grouped into three principal and well-marked divisions--the _Lamellibranchs_, or Plate-gilled molluscs, the gills of which are composed of plate-like layers, and the headless bodies enclosed in a bivalve shell; the _Cephalophora_, or head-bearing molluscs, protected by a univalve shell; and the _Cephalopoda_, or Head-footed molluscs, so called because the mouth is surrounded by tentacles or arms by which the animal can cling to objects or seize its prey. We shall deal with these three divisions in the above order, taking first the bivalves, the shells of which are found in great variety along our shores. The general nature of a lamellibranch is easily made out by the examination of one of the common species that may be obtained alive on any part of the coast, such as the Edible Mussel, the Cockle, or the Oyster, and the reader will do well to secure a few specimens and examine them with the aid of the following description of the principal distinguishing features.