240 ARTHROPODS. 



the flaps of the shield. A current of water from behind 

 forwards is kept up by the activity of the baling portion of 

 the second maxilla. Venous blood enters the gills from 

 the ventral sinus, and purified blood leaves them by the six 

 channels leading to the pericardium. 



Observed superficially, the gills look somewhat like 

 feathers with plump barbs, but their structure is much more 

 complex. The most important fact is that they present a 

 large surface to the purifying water, while both the stem and 

 the filaments which spring from it contain an outer canal 

 continuous with the venous sinus, and an inner canal com- 

 municating with the channels which lead back to the 

 pericardium and heart. 



Three sets of gills are distinguishable. To the basal 

 joints of the six appendages from the second maxillipede to 

 the fourth large limb inclusive, the podobranchs are attached. 

 They come off with the appendages when these are pulled 

 carefully away, and each of them bears in addition to the 

 feathery portion a simple lamina or epipodite. The mem- 

 branes between the basal joints of the appendages and the 

 body, from the second maxillipede to the fourth large limb 

 inclusive, bear a second set, the arthrobranchs, which have 

 no epipodites. In connection with the second maxillipede 

 there is a single arthrobranch, in connection with each of 

 the five following appendages there are two, so that there 

 are eleven arthrobranchs altogether. There remain three 

 pleurobranchs, one on the epimeron of the fifth large limb, 

 and two others quite rudimentary on the two preceding 

 segments. The basis of the podobranchs bears long setae. 



Excretory System. — A kidney or "green gland" lies 

 behind the base of each antenna, and its opening is 

 marked by a conspicuous knob on the basal joint of that 

 appendage. Each kidney consists of a dorsal sac communi- 

 cating with the exterior, and of a ventral coiled tube which 

 forms the proper renal organ. The latter is supplied with 

 blood from the antennary and abdominal arteries, and 

 forms as waste-products uric acid and greenish guanin. 

 Each kidney may be regarded as homologous with a neph- 

 ridium. 



Reproductive Organs. — The male crayfish is distingished 

 from the female by his slightly slimmer build, and by the 



