A0TAOU8 FLUVIATII.H 



G. ll.Astttcutt fluviatlli*. A. Mandible: a, b, endopodltc ; o, lt terminal joint* 

 coiiHtitutiiiL' tin- p.-tlpiiH of the mandible. B. Flrat maxilla. C. Second max 

 illn />. I 'ir-i inaxiilipedc. E. Second maxlllipede. F. Third maxilllpfde. All 

 the prvrnlini.', except //, are left limbs. G. Ambulatory letf. //. Appendage of 

 tir.-t. .-mil /of second, ahdominul somite in th! male. K. Appendage of third ab- 

 doininil somite. L. Sixth abdominal nomite, with it* appendage* and telson : 

 A, endopodite ; c, exopodite: tf. epipodlte ; . setaceous filaments attached to 

 ; ./-, ter-um of nixth abdominal somite; y, z, the two division* 

 ' 



i l-.in. in if; -i, DMlpodlte ; .'{. Uchiopodito : 4, meropodite ; 5, carpopodlte ; 



r,. propocMic ; 7, dactylopodlte. In .1. ./ marks the tendon of the adductor n. 

 .'in'! in A' Hi'- joints of a 6 and r are not ufllcienily numerous. .V. Transverse 

 section of half a thoracic somite (a): 6, coxopodite : c, basipodite; d, Isci 

 dite ; A, bnnchiflBrbai epipodiie ; /'. </, branchiae; f, filiform appendage. N. One 

 of the branchlferous cpipodites : a, its point of attachment; 6, basal enlarge- 

 ment; c, branchial filaments; '/. terminal lobes. 



The inner of these is distinguished from the outer by possess- 

 ing a more elongated and wider basal joint. The whole basal 

 division of the appendages is tho protopodite; whilr tli 

 (iM-nil and external terminal filaments are the endopodite 

 (, b) and exopodite (c). 



An abdominal segment, or somite, then, is composed of a 

 tergum, two pleura, and a sternum ; but it must be remem- 

 bered that these terms rather indicate regions tlnn anatomi- 

 cal elements, the whole segment being continuously calcified, 

 and no sutures or other absolute demarcations separating 

 one portion from another. Furthermore, the somite ca 

 two appendages, each divided into a proximal portion or pro- 

 topodite, terminated by t\vo branches, the endopodite and 

 exop lite. 



The whole exoskeleton of the Astacus, however various 

 m iv be the appearance of its different parts, consists of so- 

 mites ami jipp MI I iges essentially similar to those which have 

 just been described, but which are more or less masked by 

 the connation, the coalescence, the abortion, or the extreme 

 m ..) litication of their primitive elements. 



If, in the first place, we follow out these modifications in 

 the posterior somites, we find the fourth, fifth, and sixth 

 abdominal s unites to be, in all essential resprcis. similar t 

 the third ; but the appendages of the sixth (Fig. 71, L) are 

 singularly changed) the protnpndite being represented by a 

 single strong, short joint, and the exopOCItte sin-! ett.lopodite 

 having tin* form of wide, oval setose plates. The exopodite 

 is again divided into two portions by a tran<vers. j, >int. The 

 seventh division of the abdomen (Fig. 71, Z, y, 2) is the tel- 

 This telson bears no appendages ; dorsally it is completely 

 calcified, but is divided by a transverse suture into two 

 portions, the p->^teri-.r of which is movable upon theoilfr; 

 yentrally, on the e.Mt r.-iry. it is only the posterior part \\lii. h 

 is fully calcified, the middle of the anterior portion, in which 



