Dissected so as to show its digestive, reproductive, and respiratory systems in situ.
The greater part of the tergal region of all the segments of the body has been removed, together with the heart and its vessels and in the abdomen the thin stratum of extensor muscles. The stomach occupies a central position anteriorly, and is clearly divisible into a wider cardiac portion in front and a narrower pyloric portion behind. An arcuate plate, the 'cardiac ossicle,' crosses the cardiac portion at the point of greatest width, and receives the insertion of the major part of the anterior gastric muscles which spring from the base of the rostrum. A pyloric ossicle crosses the pyloric portion of the stomach in a similar manner, and gives attachment to the posterior gastric muscles which take origin posteriorly from the carapace. The other stomachal ossicles can only be studied when the stomach is properly opened. To the right side of the pyloric portion of the stomach is seen the end of the adductor mandibulae muscle separated from its attachment to the carapace; and behind it, as well as to either side, are the two lobes of the liver. On the left side, in front of the liver, is to be seen a small portion of the sac of the green gland. The paired anterior lobes of the testis lie in the middle line between the liver lobes. The azygos posterior lobe overlies the intestine.
At the spot where these three lobes unite the right and left vas deferens take origin as slender tubes, the calibre of which rapidly widens. They are disposed in many convolutions which intrude some way into the abdominal cavity before they turn downwards, to open on the coxopodites of the last pair of thoracic limbs. The intestine takes a straight course, as in all Crustacea, to the anus. The branchial cavity is just exposed on the left side, but largely on the right by the removal of a great part of the branchiostegite and of the united thoracic epimera which separate the viscera from the branchial cavity on the inner side. The branchiae are seen lying in the cavity thus exposed. They may be distinguished as podo-, arthro-, and pleuro-branchiae. Podobranchiae are attached one to each coxopodite from the second maxilliped to the fourth thoracic limb inclusive. The arthro-branchiae are divisible into an anterior and posterior, or external and internal set, and they are attached to the membranes uniting the coxopodites to the body. The second maxilliped bears an anterior arthrobranch only, the third maxilliped and the thoracic limbs to the fourth inclusive possess both sets. The fifth thoracic limb bears no branchiae, but a pleuro-branch is attached to the epimeron of its somite.
There are also two rudimentary pleuro-branchs, one on the third, another on the fourth, epimeral regions. The number and arrangement of the branchiae varies much among the Decapoda.
A podobranchia consists of a broad basal portion convex posteriorly and in-feriorly, beset with setae and articulated to a coxopodite. The stem of the branchia bends at right angles to this base and divides into an apical plume and a lamina. The free extremity of the plume is simple and filiform. At its base it gives origin to a number of cylindrical branchial filaments. Similar filaments spring also from the outer and anterior surfaces of the stem itself. The lamina originates about the middle of the stem. It is folded upon itself. The edge of the fold looks forwards, and the leaves of the fold are one external, the other internal, the latter extending downwards towards the base of the stem to a distance greater than the former. The folded edge of one podobranchia fits into the space between the leaves of the foregoing podobranchia. The surface of each leaf is plaited longitudinally ten or. twelve times. The edges and surfaces of the leaves, especially of the plaits, are beset with small elevations each bearing a single minute hooked seta. The epipodite (so-called) of the first maxilliped represents the base, stem and lamina of a podobranch.
It is slightly folded, but the edge of the fold is posterior, whilst the internal edge of the lamina and its posterior surface bear hooked setae. The structures known as coxopoditic setae are long and slender filaments arising close to the bases of the podobranchiae, with acute apices and their terminal portions beset with foliaceous scales. They ascend vertically, lying among the branchiae, and it is suggested by Professor Huxley that they exclude parasites. The Crayfishes (Paras-iacidae) of the S. Hemisphere differ from those (Potamobiidae) of the N. Hemisphere in having (1) the laminae of the podobranchs rudimentary; (2) some at least of the branchial filaments, the setae of the stem, and the coxopoditic setae terminally hooked; and (3) a few branchial filaments upon the epipodite of the first maxilliped. An arthro-and pleuro-branch have a structure similar to the base, stem and apical plume of a podobranch. The anterior of the two rudimentary pleurobranchiae is often a mere papilla, but the posterior resembles in structure a branchial filament. Of the two varieties (? species) of A. fluviatilis, the A. nobilis differs from A. torrentium in possessing three instead of two rudimentary pleurobranchiae.
The Crayfishes of the S. Hemisphere, with the exception of Astacoides from Madagascar, have four functional pleurobranchiae. It is remarkable that these organs are entirely absent in Cambarus (the only other genus besides Astacus of Potamobiidae) which is distributed East of the Rocky Mountains from the Great Lakes to Guatemala, and is found also in Cuba.
Inasmuch as the branchial filaments are all cylindrical in the Crayfishes, the branchial plumes are tricho-branchiae. The Crayfishes in this respect agree with all Decapoda Macrura except the genera Gebia and Callianissa, the Prawns, Shrimps, and Mysidae. The branchiae of the last-named are either absent or rudimentary. In the other Macrurans mentioned, as in the Hermit Crabs and Brachyura, the filaments are replaced by lamellae, and the branchiae are phyllo-branchiae.