264 CRUSTACEA. 



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 membranes 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 

 pleurobranckSy one on the epimeron of the fifth large limb, and two 

 others quite rudimentary on the two preceding segments. The bases 

 of the podobranchs bear long setae. 



In Nephrops and the common lobster the number and arrangement of 

 the gills is slightly different. 



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 communicating 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 nephridium. 



In Palcemon, the kidney is connected by a glandular duct 

 with a delicate dorsal "nephro-peritoneal sac," possibly coelo- 

 mic. more probably an enlargement of the nephridial system. 



The crayfish has also, near the gills, small branchial glands which 

 excrete carcinuric acid from the blood, and also help in phagocytosis, 

 that important process in which wandering amoeboid cells resist infection 

 and help to repair injuries (cf. possible function of thymus in Fishes). 



Reproductive Organs. 



The male crayfish is distinguished from the female by his 

 slightly slimmer build, and by the copulatory modification 



