XXX11 



THE VOYAGE OF H.M.S. CHALLENGER. 



thickened condition of the margin. Both the specimens in which this condition has 

 been observed are males. It is not impossible that it may have been produced by 

 rubbing against objects during pursuit of the female, but in this case we should expect 

 some similar condition in other genera, and I know of none. 



I have recently had the opportunity of seeing the extensive and well-preserved series 

 of specimens collected by Professor A. Milne-Edwards during the voyage of the 

 " Talisman," 1 among which he drew my attention to a species of Aristeus, 

 in which the scaphocerite of all the adult males had the foliaceous 

 extremity produced in length to a considerable degree (Fig. X.). It 

 would seem as if this condition might be valuable as not interfering with 

 the speed of the male when in chase of the female, and perhaps of 

 grasping her when caught. 



Figure c on PL L. shows the scaphocerite previous to its having 

 undergone much change ; but a slight emargination, which is not a con- 

 stant feature, demonstrates the area in which the abnormal thickening 

 takes place in older male specimens. 



In some of the younger stages, such as may be seen in those of 

 Sergestes {Elaphocaris crassus, PI. LXI. fig. 4c ; Platysaccus crenatus, 

 PL LXIII. ; and Elaphocaris, pp. 354, 359), and in the Zoea of Alphcus 

 (PL LXXXIX. fig. 4e), the scaphocerite exists as a cylindrical multiar- 

 ticulate appendage, fringed with ciliated hairs attached to each articulus 

 on one side only, which demonstrates its homotypical relation with 

 the basecphysis of the pereionic and pleonic appendages. From this 

 condition it gradually passes into the uniarticulate squamose plate of the normal 

 scaphocerite, the only exception being in the genus Atya, in which a diaeresis crosses 

 the middle of the scaphocerite (PL CXVI.). 



The third joint of the peduncle in all Macrura articulates with the second by two 

 corresponding tubercles, one on the inner, and one on the outer margin ; the inner 

 being considerably the more advanced, gives the articulation an oblique direction. 



The fourth joint articulates with the third by similar processes on the upper and 

 lower margins ; and the fifth joint articulates with the fourth by processes on the inner 

 and outer sides. Thus the peduncle is capable of being moved in every direction by 

 the powerful muscles situated at the base, the range of movement being considerably 

 increased by the alternating articulations, and the correspondingly alternate positions of 

 the muscles of each succeeding joint. 



The fifth joint is generally short, constantly anchylosed with the fourth, or so rigidly 



1 I cannot here pass over the opportunity of acknowledging the great courtesy of Professor A. Milne-Edwards in 

 sending over to me at Plymouth the beautiful specimen in order that I might have the opportunity of making the 

 drawing from which the above figure was taken. 



Fig. X.— Aristeus 

 coralinus, A. 

 Millie-Edwards. 

 Scaphocerite. 



