Dissected so as to show its nervous, digestive, circulatory, and reproductive systems in situ; the various organs having been exposed in a vertical section, by the removal of the tegumentary skeleton, the muscles, and the hepatic lobes of the left side. See Prep. 33, p. 177, ante.

Common Crayfish, Astacus fluviatilis.

Plate X. Common Crayfish, Astacus fluviatilis.

a. OESOPHAGUS leading vertically upwards from the mouth into the stomach. The labrum, the free edges of the mandibles, and of the two maxillae, are faintly indicated on the right side of the mouth anteriorly to the three foot-jaws.

b. Cardiac portion of stomach. Superiorly and anteriorly the stomach is still retained in its natural position, the anterior gastric muscles, which took origin superiorly to the supra-oesophageal ganglia, 0, from the under surface of the ventral wall of the hollow rostrum, and attached themselves to the cardiac plate, having been left intact; whilst the anterior wall of the stomach has been displaced a little backwards in order to give a better view of the stomato-gastric nerves.

c. Lateral prominence of pyloric filter.

d. Hepatic lobes of right side.

e. Orifice by which the hepatic lobes of the left side opened into the digestive tract.

f. Intestine passing with the straight course characteristic of Crustacea, with the exception of Lynceus, to the anus.

g. Anus, opening on the inferior surface of the 'telson ' in uncalcified membrane, anteriorly to the junction of its anterior and posterior halves. h. Heart, showing one of the lateral venous orifices, and the posterior aorta which divides into two main branches, i and j. i. Post-abdominal artery, taking a course superiorly to the intestine, and inferiorly to the extensor muscles of the posterior somites. j. Sternal artery passing down to the interval between the commissural cords connecting the third and the fourth abdominal ganglia. k. Hepatic artery of the left side, passing from the heart over the pylorus towards the left hepatic lobes which have been removed. l. Anterior left lobe of testis.

l'. Azygos lobe of testis placed behind the paired lobes

.m. Convolutions of vas deferens of left side, in length equal to that of the entire body. The agglutinating matter of the spermatophores is probably secreted in them. n. Coecal sac of mesenteron or archenteron.

0. Supra-oesophageal ganglionic mass.

p. First post-oral ganglion, supplying the mandibles, the two pairs of maxillae, and the three pairs of foot-jaws, or thoracic appendages. In the developing Astacus this mass consists of six pairs of ganglia, in correspondence with the six sets of appendages it innervates. In Insects, the first post-oral ganglion is always distinct from the thoracic ganglia, whilst in other Arthropoda it is generally fused with more or fewer of them.

q. Median stomatogastric nerve formed by the fusion of four nerves, two from each oesophageal ganglion, to q'.

q'. Azygos nerve, passing downwards from the middle of the posterior edge of the supra-oesophageal mass to meet four nerves given off, two from each of the two oesophageal ganglia, developed upon the commissural cords of the nerve-collar. It forms with them the nerve, q. The nerve from the anterior edge of the supra-oesophageal ganglion is not shown. Cf. p. 189, ante.

r. Fifth thoracic, or sixth post-oral ganglion. Posteriorly to it are the six abdominal ganglia. From this ganglion and the two in front of it, long nerves pass off upwards to the reproductive organs and the superiorly placed muscles.

s. Multi-articulate flagellum or endopodite of second pair of antennae.

t. Exo- and endopodite of first pair of antennae or antennules.

u. Second joint of third thoracic appendage, 'maxilliped', or 'foot-jaw.' This joint, as in the forceps v 1, next behind it, represents two joints, the basipodite and the ischiopodite of the normal seven-jointed endopodite, seen in v 2, v 3, v 4, v 5. Its antero-internal edge is denticulate, as in the Lobster, but its serratures are concealed by a fringe of setae.

v 1. Fourth thoracic appendage, or forceps, modified by the production of the distal outer angle of the propodite so as to form a pair of pincers with the opposed last joint, or dactylopodite. Two other joints, the 'carpopodite' and 'meropodite,' are shown in this figure.

v 2 and v 3. Fifth and sixth pairs of thoracic or first and second ambulatory legs, differing from v 1 in their smaller size, and in not having the second and third joints fused.

v 4 and v 5. Seventh and eighth pairs of thoracic or third and fourth ambulatory legs of the Decapod. The two terminal joints do not form pincers, otherwise they resemble v 2 and v 3. The vas deferens opens in the coxopodite or basal joint of v 5.

w 1 and w2. The appendages of the two first abdominal somites modified so as to form an accessory copulatory organ in this a male specimen.

w 3, w 4, and w 5. Appendages of the third, fourth, and fifth abdominal somites, consisting each of two basal joints, which serve as a pedicle to two multiarticulate filaments, an exopodite and an endopodite.

w 6. Appendage of sixth abdominal somite, forming the right lateral element of the caudal fin. The telson is interposed mesially between the two appendages of the sixth abdominal somite.

x. Flexor muscles acting on the swimmeret and abdominal somites in the animal's rapid movements; its slower movements being dependent upon the ambulatory feet.

y. Extensor muscles, in two layers like the flexors, but of much smaller size.