16 THE VOYAGE OF H.M.S. CHALLENGER. 



In most other species the same conditions occur, and a comparison in detail of the 

 posterior epimera, which difler more in length in the two sexes than the epimera of the 

 anterior segments, will be found under the description of species. 



Some few species do not show these differences; in Serolis paradoxa, for example, the 

 males, on a superficial view, are indistinguishable from the females, and the difference in 

 size between the two sexes is hardly if at all marked. 



As a general rule the sterna of the three anterior abdominal segments serve to 

 distinguish the sex of the individual ; in the females the middle portion is commonly 

 prolonged into a stout spine, while in the males this structure is not present, and 

 the posterior margin of the segments is straight or slightly concave. In a great 

 number of species, however, the two sexes do not differ at all in this way. 



Another marked secondary sexual character, which is quite univei^sal in the form of 

 the third thoracic appendages ; in the females this pair of appendages is entirely similar 

 to the succeeding ambulatory limbs ; in the males, on the contrary, the penultimate joint 

 is swollen and furnished on the inner side wdth a number of peculiar modified spines, 

 the terminal joint is recurved, and the appendage thus forms a prehensile organ very 

 like the second thoracic appendage. Of Serolis tuherculata Grube states {loc. cit., 

 p. 230) — " Die Fiisse des 2'™ Fusspaares sind weniger ausgepriigte Greiffiisse als bei 

 andern Arten ; zwar zeichnet sich das 3'* und 4"^ Glied durch seine Kiirze vor den ent- 

 sprechenden der folgenden Beine aus, aUein das Handglied ist weniger breit als sonst, sein 

 Innem-and nicht liings der ganzen Schneide mit Ziihnen besetzt, und die Klaue scheint 

 nicht so zum einschlagen geeionet zu sein. Die Ziihne sind ziemlich lanof und stachel- 

 formig mit einer Andeutung von Nebenzacken." The male specimen of this species 

 which I have examined myself does indeed display such differences from the ordinary 

 structure of these appendages in all other Serolis as Grube describes ; both 

 specimens are, however, evidently immature ; the characters that he mentions exactly 

 correspond to the appendages of immature males (see infra, p. 27). 



These appendages are used liy the male during coj)ulation ; the claw is firmly 

 imbedded in the epimera of the female, so firmly that the individuals can hardly Ix; 

 separated without injury.^ 



In all species of Serolis, as in many other Isopoda, the second pair of abdominal 

 appendages bear a long penial filament ; these are a continuation of the lower margin 

 of the eudopodite of the limb, and reach in some cases as far back as the end of the 

 caudal shield ; in other species they are not quite so long. The end of these filaments is 

 blunt and rounded, and not furnished with any aperture ; it seems possible for this reason 

 that the two are approximated during copulation, and form a groove down which the 

 spermatophores pass ; the fact of the male generative apertures being placed so closely 

 together seems to favour this supposition. 



' StuJtr, Isopoden gesammelt wakreud der Reise, &c., loc. cit. 



