CRAYFISH 107 



the perivisceral sinus surrounding the alimentary canal carries 

 absorbed food materials as well as metabolic wastes. 



The next step is the passage of the blood from the sternal sinus 

 through the vessels in the gills. These important respiratory 

 organs of the Crayfish are attached, as noted below, either by 

 means of the epipodites to certain appendages or to membranes 

 which are present near the base of the appendages. After passing 

 through the gills, the oxygenated blood flows dorsally through a 

 number of canals (branchio-cardiac canals) which lead into the 

 pericardial sinus, from which it is drawn into the heart through the 

 ostia, and thus the cycle is completed. 



3. Respiratory System 



In the Earthworm it was noted that the entire surface of the 

 body acted as a medium through which the blood, plentifully 

 supplied to the body wall, could exchange its waste carbon dioxide 

 for the essential oxygen. In the Crayfish this phase of respiration 

 is performed by a much more elaborate mechanism consisting of 

 the branched, filamentous gills present in the gill chamber of the 

 thorax. The gill chamber, on either side of the thorax, lies outside 

 of the body wall. It is the space between the outer, chitinous 

 exoskeleton and the true body wall of the thorax, dorsal to the 

 attachment of the appendages. A current of water is forced 

 through each gill chamber by the paddle-shaped scaphognathite 

 which, as previously noted, is a part of the second maxilla at the 

 anterior end of the gill chamber. (W. pp. 161-162.) 



The gills in the common American species, Cambarus a ffinis, are 

 attached either to the coxopodites of the appendages (podo- 

 branchiae) or to membranes developed at the bases of the ap- 

 pendages (arthrobranchiae). In the European genus Astacus 

 another row of gills (pleurobranchiae) is present which are 

 attached to the walls of the thorax. The number of gills varies in 

 different species, but in Cambarus there are 17 gills in the gill 

 chamber on each side of the thorax : six are podobranchiae, and 

 eleven are arthrobranchiae. 



Each gill may be described as a plume-like structure with a 

 central stem, or epipodite, to which a large number of fine fila- 

 ments, the branchiae, are attached. Running through the stem 

 and branching into the filaments are two blood vessels, an effer- 

 ent branchial vessel which carries blood into the gills from the 



