Chilton. — 0?? Astacus fiuviatilis and Paranephrops setosus. 163 



Olfactory organs. — These are borne on the exopodite of the antennule. 

 The various joints in the flagellum differ in shape from those of Astacus, 

 for the distal end of each is considerably enlarged ; this enlargement is 

 chiefly on the under side, for, while the upper edge is nearly straight, the 

 under edge curves so as to form a rounded protuberance towards the distal 

 end (pi. XXI., fig. 1). On these protuberances are situated the tufts of 

 olfactory setaa (c), thus there is only one tuft of olfactory setaa on each 

 joint, wbile in Astacus there are two on each joint. Each tuft arises 

 from a slight cavity in the joint and consists of usually five or six olfactory 

 setaa. 



The olfactory setaa are similar in shape to those of Astacus. Each con- 

 sists of two parts (pi. XXI., fig. 2), which at first sight appear to be divided 

 by a joint, but on more careful observation it appears that this is not really 

 so, but that the walls of the two parts are continuous, and that the appear- 

 ance of a joint is caused by the basal part being filled with opaque granular 

 matter while the distal part is clear and transparent. This opaque gran- 

 ular matter extends up the sides further than it does in the middle of the 

 seta. 



Reproductive organs. — Male (pi. XX., fig. 7). The testes (t) consist of 

 two long tubes united towards their anterior ends by a transverse portion or 

 commissure. The two parts in front of this commissure lie between the 

 heart and the posterior portion of the stomach, and are somewhat directed 

 upwards towards the dorsal surface of the body. Immediately in front of 

 the commissure they become considerably narrower. The portions of the 

 tubes behind the commissures are narrow at first but they soon widen and 

 then soon contract again at the origin of the vasa deferentia. After this they 

 again widen out and at their posterior ends are more closely approximated. 

 Thus a little behind the commissure a considerable space is left between the 

 two tubes, and in this space the heart rests. The vas deferens (yd) arises 

 as a very fine tube. The first part differs from the remainder in being 

 smaller and less boldly curved. The remainder is exceedingly convoluted 

 and increases only very slightly in size until it comes to the portion which 

 proceeds directly downwards to the aperture on the bases of the last pair of 

 ambulatory legs. This (a) is enormously expanded and is not quite 

 cylindrical, being somewhat laterally compressed. In the figure the con- 

 volutions have been separated. I have not been able to observe the 

 spermatozoa, but I have seen in some specimens the other substance 

 which Professor Huxley mentions as filling the vasa deferentia together 

 with the spermatozoa. As in Astacus it was of a viscjd material and 

 gave " the secretion of the testis the form and consistency of threads of 

 vermicelli." 



