200 On the Sexual Characters of Ligia oceanica. 



On comparison of this with fig. 2 prp 2 , which represents the 

 same appendage in the female, it will be seen that in the male 

 the meros, carpus, and propodos are all produced on the inner 

 side into a flat plate-like expansion, with the free border 

 more or less convex and fringed with short setae ; on the 

 anterior surface this expansion is on a level with the rest of 

 the joint, but on the posterior aspect (shown in fig. 1 prp 2 ) 

 the cylindrical portion of each joint can be seen and the plate- 

 like character of the expansion thus rendered more evident. 

 On examination with a high power each expansion shows on 

 its surface rows of small serrations, giving it an appearance 

 like that of a file ; and further magnification shows that this 

 is due to closely-set rows of minute setae. In the female 

 there is no trace of these expansions, and the inner border of 

 the different joints, more especially of the meros and carpus, 

 bear numerous stiff seta;, which are larger and more irregular 

 in size than those found in the corresponding positions in the 

 male. 



In order to make this short paper on the sexual characters 

 of Ligia oceanica somewhat more complete, I have given 

 figures of the first and second pleopoda of the male. These 

 have been already drawn and briefly described by Sara *, 

 and as their form can be readily made out from the figures, a 

 detailed description is not here necessary. In the second 

 pleopod (PI, VIII. fig. 1 pip 3 ) the whole of the endopodite 

 appears to be modified into a two-jointed styliform organ, 

 moved by powerful muscles ; its second joint is long and 

 cylindrical, and along with the external male organ proper 

 (which is figured in connexion with the first pleopod in 

 fig. 1 pip 1 ) no doubt forms a channel for the passage of the 

 spermatozoa ; its extremity is roughened on the inner side from 

 the presence of numerous closely-set short setae. 



In both pleopods is seen a more or less oval plate, external 

 to the exopodite and arising apparently from the outer part of 

 the basal portion of the pleopod ; in the case of the second 

 pleopod its margin is fringed with fine setae. This plate has 

 been figured by Sars in this and in other species of the 

 Oniscoidea, but I cannot find any special reference to it, and 

 I am not quite certain as to its exact homology and signifi- 

 cance; it appears, however, to correspond with a similar plate 

 found on the third, fourth, and fifth pleopoda in the species of 



* ' Crustacea of Norway/ II. Isopod.i, p. 155,' pi. lxx. figs. pW <$ and 

 v¥ J. 



