A small part of the intestine at the hinder end is lined, like the stomach, by a continuation of the chitinous covering, which is turned in at the vent. This lining and that of the stomach, with the plates and teeth of the gastric mill, are cast and renewed when the shell is moulted.
On each side of the food-canal in the thorax lies a large mass of soft tissue, yellowish-green in colour. This is the digestive gland, or "liver," which secretes the digestive juice, discharging it into the food-canal by a short duct on each side just behind the stomach.
The heart lies in the middle of the back, just under the hinder part of the carapace, and gives off, in front and behind, a number of arteries which carry the blood to the various organs of the body. From the smaller branches of these arteries the blood passes, not, as in vertebrate animals, into capillaries, but into the spaces lying between the organs of the body, and it finds its way back to the heart, not in definite veins, but by ill-defined venous channels which open into the pericardium, or space surrounding the heart. From the pericardium the blood enters the heart by six openings in its walls, each guarded by a pair of valves which close when the heart contracts, and prevent the blood from returning to the pericardium.
Fig. 7—Gills of the Lobster, exposed by cutting away the Side-flap of the Carapace (Branchiostegite)
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The venous channels which convey the blood back to the heart are so arranged that most of the blood passes first through the gills, for the purpose of respiration, before it reaches the heart and is again distributed through the body. These gills, as already mentioned, lie in the two branchial chambers under the side-flaps of the carapace ([Fig. 7]), and are attached, some to the epipodites of the thoracic limbs (as described above), and some to the soft membrane of the joints between the limbs and the body; while others are attached to the side-wall of the thorax itself. Each gill is somewhat like a bottle-brush in shape, consisting of a central stalk set round with rows of soft hair-like processes. As the blood streams through the minute channels inside these filaments, it is separated only by a thin membrane from the surrounding water, and the absorption of oxygen and discharge of carbon dioxide can go on easily. For this purpose, however, it is necessary that the water within the gill chamber should be constantly renewed, and this is effected in the following way: the front part of the gill chamber forms a narrow channel running forward under the side-wall of the carapace. Within this channel lies a large plate known as the scaphognathite, attached to the outer side of the maxilla, which during life is constantly in movement, causing a current of water to flow forwards through the channel. The water enters the gill chamber by the narrow slit-like space between the lower edge of the carapace and the bases of the legs, and is discharged in front at the sides of the head, where its movement is helped by the vibrating exopodites of the maxillipeds.
At the sides of the stomach, in the front part of the head, lie a pair of glands which, from their colour, are known as the green glands. These are the excretory organs, corresponding in function to the kidneys of the higher animals. Each has connected with it a thin-walled bladder, which opens to the outside through a small perforation on the under-side of the first segment of the antenna.
The chief part of the nervous system is the ventral nerve-chain, which runs along the under-side of the body. This is a long cord having at intervals a series of knots or swellings, the ganglia or nerve-centres, from which nerves are given off to the appendages and to the organs of the body. In the hinder part of the thorax and in the abdomen there is a ganglion in each somite, but in front these ganglia become crowded together and coalesced, so that we find only a single large ganglion, corresponding to the somites from that of the mandibles to that of the third maxillipeds. Between the ganglia the cord is really double, although for the greater part of its length the two parts are more or less completely fused into one. In front of the head and above the gullet is a ganglion which sends nerves to the eyes, antennules and antennæ, and is known as the brain, although it is, perhaps, hardly so important as that name would suggest. It is connected with the ventral chain by two cords that pass on either side of the gullet.