applied in Crustacea are by no means exactly equivalent to those which bear the same names in Insects, for example, and that, beyond a rough similarity in position, they have no sort of relation to the parts so named in the body of a vertebrate animal. [2] The account given here of the structure of the Lobster applies almost equally well to the River Crayfish or the Norway Lobster. The student is recommended to follow the description with a specimen of one of these animals before him. [Illustration: FIG. 1--THE COMMON LOBSTER (_Homarus gammarus_,) FEMALE, FROM THE SIDE. (From British Museum Guide.)] There are altogether twenty pairs of appendages attached to the body of the Lobster. In front of the head are the stalked _eyes_ (of which the nature will be discussed later) and two pairs of feelers--the _antennules_ and _antennæ_ (sometimes called the first and second antennæ). Near the mouth on the under-side of the head are three pairs of jaw-appendages--the strong _mandibles_ and the flattened, leaf-like _maxillulæ_ and _maxillæ_. Following these are the appendages of the thorax, of which the first three are intermediate in form between the true jaws and the legs, and are therefore termed foot-jaws, or _maxillipeds_. The remaining five pairs of thoracic limbs are the _legs_, the first pair forming the large and powerful pincer-claws, or _chelipeds_, while the others are the walking legs. The six pairs of swimmerets on the abdomen have already been mentioned. If one of the somites of the abdomen be separated from the others, it will be seen (Fig. 2) to consist of a shelly ring, to which the two swimmerets are attached, wide apart, on the under-side. The arched upper part of the ring is known as the _tergum_, and the more flattened under-part as the _sternum_. On each side the tergum overlaps the sternum, and hangs down as a side-flap, or _pleuron_. On the upper side of the abdomen the terga of the somites overlap, the front part of each being pushed under the tergum in front when the abdomen is straightened, and only exposed to view when the abdomen is bent. Below, the sternum of each somite is seen to be only a narrow bar, connected with those in front and behind by soft membrane, and there is no overlapping. At the sides the somites are connected together by hinge-joints, which allow them to move only in a vertical plane. Thus the abdomen can be straightened out or bent downwards and forwards, but cannot be moved from side to side. In life the Lobster can swim backwards through the water by vigorously flapping the abdomen. [Illustration: FIG. 2--ONE OF THE ABDOMINAL SOMITES OF THE LOBSTER, WITH ITS APPENDAGES, SEPARATED AND VIEWED FROM IN FRONT. (From British Museum Guide.)]