210 Description of the Plates. 



w 6. Appendage of sixth post-abdominal segment, forming the 

 right lateral element of the swimmeret. It consists of a 

 single basal joint, supporting a biarticulate squamiform exo- 

 podite and a uniartieulate eudopodite^ which reverse thus 

 the positions of relative size held by these elements in the 

 segments anterior to the swimmeret. The telson is interposed 

 mesially between the two appendages of the sixth post- 

 abdominal segment. 



X. Flexor muscles acting on the swimmeret and post-abdominal 

 segments in the animaFs rapid movements ; its slower 

 movements being dependent upon the ambulatory feet. 



y. Extensor muscles, in two layers like the flexors, but of 

 much smaller size. 



For a full account of the anatomy of the Crayfish, see Huxley, 

 Medical Times and Gazette, Feb. 7, 1857, et seqq. 



For figures of the various systems and organs of the animal, see 

 Brandt, Medizinische Zoologie, Bd. ii., tab. xi., pp. 62-64. 



For the stomatogastric system, see Brandt, Ann. Sci. Nat., Ser. ii., 

 torn, v., 1836, pi. iv., figs. I, 2, pp. 87-91. 



For the olfactory (auditory?) organ, as carried by the internal or 

 superior pair of antennae, see Gerstaecker, Bd. v., p. 357, and 

 La Valette, Leydig, and Fritz MiiUer, cit. in loc. See also 

 Spence Bate, Sessile-Eyed Crustacea, vol. i., p. ix. 



For the formation of the coecal sac opening into the commencement 

 of the duodenum out of the yolk-sac, see Rathke, Entwick- 

 elungsgeschichte der Flusskrebses, p. 64. For other coecal 

 appendages to the digestive tract of Crustacea, see Milne Ed- 

 wards, Hist. Nat. des Crustaces, 1834, pp. 76, 77; and for the 

 formation of the liver by a bilateral out-pouching of the yolk- 

 sac, see Rathke, I. c. p. 49, and Abhandlungen zur Buldungs-und 

 Entwickelungsgeschichte, Theil. i., 1832, p. 15 ; Theil. ii., 1833, 

 p. 78, in which latter memoir the relation of intestinal tract and 

 yolk-sac is shown to be diff'erent in Oniscus and Astacus. 



