ASTACUS FLUVIATILIS. 2G7 



Fig. Il.—Astacusfiuviatilis. A. Mandible: a, b, endopodite ; o, its terminal joints 

 constituting the palpus of the mandible. B. First maxilla. C. Second max- 

 illa. D. First maxillipede. E. Second maxillipede. F. Third maxillipede. All 

 the preceding, except B, are left limbs. G. Ambulatory leg. //. Appendage of 

 first, and /of second, abdominal somite in the male. K. Appendage of third ab- 

 dominal somite. L. Sixth abdominal somite, with its appendages and telson : 

 a, 6, endopodite ; c, exopodite ; d, epipodite; e, setaceous filaments attached to 

 coxopodite ; x, tergum of sixth abdominal somite; y, z, the two divisions of the 

 t j lson. In G: 2, basipodite ; 3, ischiopodite ; 4, meropodite ; 5, carpopodite ; 

 6, propodite ; 7, dactylopodite. In .4, d marks the tendon of the adductor muscle, 

 and in K the joints of a b and c are not sufficiently numerous. M. Transverse 

 section of half a thoracic somite (a): b, coxopodite : c, basipodite; d, ischiopo- 

 dite ; h, branchiferous epipodite ; /, g, branchiae; e, filiform appendage. N. One 

 of the branchiferous epipodites : a, its point of attachment; b, basal enlarge- 

 ment; c, branchial filaments; d, 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 the protopodite ; while the in- 

 ternal 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 than anatomi- 

 cal elements, the whole segment being continuously calcified, 

 and no sutures or other absolute demarcations separating 

 one portion from another. Furthermore, the somite carries 

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

 topodite, terminated by two branches, the endopodite and 

 exopodite. 



The whole exoskeleton of the Astacus, however various 

 may be the appearance of its different parts, consists of so- 

 mites and append tges essentially similar to these which have 

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

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

 modification 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 somites to be, in all essential respects, similar to 

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

 singularly changed, the protopodite being represented by a 

 single strong, short joint, and the exopodite and endopodite 

 having the form of wide, oval setose plates. The exopodite 

 is again divided into two portions by a transverse joint. The 

 seventh division of the abdomen (Fig. 71, X, ?/, z) is the telson. 

 This telson bears no appendages ; dorsally it is completely 

 calcified, but is divided by a transverse suture into two 

 portions, the posterior of which is movable upon the other; 

 ventrally, on the contrary, it is only the posterior part which 

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



