Common Crayfish. 95 



correspond to the appendages already described in the five ante- 

 rior post-abdominal segments^ and consist each of an uniarticulate 

 * protopodite/ which carries on its apex a biarticulate ' exopodite' 

 and a nuiarticulate ' endopodite/ The mesial element of the 

 ' swimmeret^ is constituted by the so-called ' telson/ an azygos 

 plate divided in the crayfish into an anterior and posterior portion 

 by a transverse suture, immediately anteriorly to which the anus 

 opens on the ventral surface of the body. The telson has some- 

 times been reckoned as a seventh post-abdominal segment, but 

 as it, with scarcely an excej)tion, is without appendages ; as it is 

 generally aborted, or rudimentary, or fused with the sixth segment 

 in Hedriophthalmata ; and as, finally, it is developed after the other 

 segments and from the dorsal surface of the body, it is better to re- 

 gard it as being simply an outgrowth from the sixth segment of the 

 post-abdomen, in the same way as the rostrum may be considered to 

 be an outgrowth from the carapace. The proximal segment of the 

 'telson'' is not calcified continuously across its ventral surface, 

 whereas the true post-abdominal segments have each of them a 

 horizontal calcified chitinous chord, connecting, without the inter- 

 position of any suture, the opposed internal surfaces of the arc 

 represented by their dorsal wall. At the junction of the chord 

 and arc are the articular surfaces for the post-abdominal append- 

 ages already described. The portion of the dorsal wall which 

 is prolonged downwards beyond the level of the junction of the 

 ventral and dorsal portions of the external skeleton of each segment 

 is known as the ' pleuron.'' There is no such element in the telson. 

 The pleura of the sixth, fifth, fourth and third segments, and the 

 convex tergal surfaces whence the pleurae arise in all the segments, 

 have facets developed upon them anteriorly, which are overlapped 

 by processes of the segments next in front. The pleura, however, 

 of the second and first post-abdominal segments, develope processes 

 which are not overlapped by, but themselves overlap, the one the 

 pleuron of the first post-abdominal segment, and the other the 

 posterior pleural edge of the carapace. 



A quadrangular area, corresponding pretty accurately with the 

 position of the subjacent heart, is marked off in the posterior portion 

 of the carapace, or ' omo-stegite,^ by two linear depressions on 

 either side the middle line, and by an anterior faintly marked line, 

 curving concentrically with the convexity of the cervical suture, 



