Young Stages o/Gecarcoidea lalaiidii. 533 



the modification, viz. tlie small size of the fifth thoracic leors, 

 their dorsal position, and terminal group of three long set«, 

 may often occur singly as separate characters, the inegalopse 

 of certain species of Portunus, for example, having the last 

 thoracic leg tipped with three long curved setaj, but the limb 

 not reduceil in size nor markedly dorsal in position. 



I have been unable to find any record of a Megolopa in 

 which this modification is so pronounced as in G. lalandii. 



It is difficult to see of what use this leg can be to a free- 

 swimming larva such as tiie Megalopa, unless it is used to 

 clean the long-fringed seta} of the pleop )ds, which tend to 

 become clogged with small fragments as the animal swims 

 through the water. If this is actually the case, it would 

 account for the fact that in the next stage, when the pleopods 

 nre no longer used for swinniiing, the fifth leg is once more 

 normal in .structure and position. 



Another point of interest is the development of the abdo- 

 minal appendages or pleopod.^, in connection with the stao-e 

 of development at which sex-ditfereutiation becomes exter- 

 nally a])parent. 



In the adult male of this species the appendages of the 

 first and second abdominal segments only are present, and 

 thi;se are modified as copulatory organs. The adult female 

 has four pairs of biramous pleopods, from the second to the 

 iit'th segment inclusive, and in neither sex is there any trace 

 of the uropods. The male may also be distinguished by the 

 possession of larger chelaj and a narrower abdomen than 

 those of the female. 



in the megalopa and young stage here described the two 

 sexes cannot as yet be distinguished on external examination. 

 The megalopa has four pairs of biramous pleopods, from the 

 second to the fifth abdominal segment ; the outer ramus a 

 flattened oval, thickly bordered with plumose setae ; i\\Q 

 inner a blunt process with a group of coupling-hooks at the 

 end of its inner margin. These are linked into similar hooks 

 on the corresponding pleopod of the other side, so that in 

 swimming the two members of a pair move as one. The 

 uropods have no inner ramus, but the outer ramus is well- 

 developed and fringed with plumose seta?. 



In tiie subsequent young stage, the four pairs of pleopods 

 are all reduced to .spineless processes, of which only the first 

 two pairs show any trace of a biramous character, while the 

 uropods, though still present, are quite vestigial. This 

 reduction is apparently carried out to the same extent in both 

 sexes, so that in order to attain the condition present in the 



Ann. (C- Mag. N. Hist. Ser. 9. Vol x. 3r. 



