1 82 DESCRIPTIONS OF PREPARATIONS. 



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 arrange- 

 ment 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- 

 tacidae) of the S. Hemisphere differ from those (Potamobiidae) of the N. Hemisphere 

 in having (i) 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. torren- 



