XII] ASTACUS 187 



antenna-like organ grows in its place, suggests that after all the 

 eye-stalk may be an appendage which we might perhaps compare to 

 the "antennae" on the prostomium of a Polychaete worm. 



The crayfish like all Crustacea is primarily adapted to a water- 

 breathing life, though like many members of its class it is able to 

 pass short periods out of the water if its respiratory organs be kept 

 moist. These organs are feathery outgrowths of the body and limbs 

 termed gills or branchiae. By cutting away the branchiostegite of 

 one side the structure and arrangement of these organs can be 

 observed. Commencing behind we find in the English crayfish 

 (Astacus fluviatiUs) a small gill springing from the thin side- wall 

 of the thorax high above the insertion of the last leg. This gill, 

 from its place of origin termed apleurobranch (Gr. irXtvpov side), 

 is absent in the species of Cambarus. It has the form of a fila- 

 mentous stem beset with small flattened branches on all sides, 

 through the thin walls of which the oxygen dissolved in the water 

 diffuses inwards to the blood. 



Proceeding forwards we find above the other thoracic legs no 

 pleurobranchs (though in the Crabs [Brachyura] there is quite 

 a series of them), but attached to the arthrodial membrane which 

 connects the fourth leg to the body there are two gills, one placed 

 above the other, which from their position may be termed arthro- 

 branchs. These gills are similar in structure to the pi euro - 

 branch, but there is in addition a gill attached to the coxopodite 

 of the limb, of rather a different structure. It commences in a 

 circular basal plate from which springs a flattened stem called the 

 lamina beset with two series of branches. This gill is termed 

 a podobranch because it springs from the limb itself. 



The 4th, 3rd, and 2nd legs, the chela or 1st leg and the 

 third maxillipede have each attached to their bases and arthrodial 

 membranes, a podobranch and two arthrobranchs, the second 

 maxillipede has only a podobranch and the lower arthrobranch, 

 whilst attached to the base of the first maxillipede there is a 

 structure called an epipodite, which is a lamina devoid of gill 

 filaments and must be regarded as a rudimentary gill. Hence 

 if we construct a chart showing the arrangement of the gills 

 of the English crayfish, we find remains of four parallel series 



of gills y^ll \ \ " . The extent to which each series 



,/Q e ___ e / 



is developed really depends on the shape of the branchial cavity, 



