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



much stouter than its endopodite, and forms an " operculum " divided by a transverse 

 suture, which may be at right angles to the longitudinal axis [Serolis tuberculata, &c.) or 

 inclined more obliquely to it (Serolis schythei, &c.) ; very commonly {Serolis necera, PL V. 

 fig. 1 1) the endopodite of this appendage is bifid at the tip, the inner branch being prolonged 

 beyond the outer ; in most cases, however, the endopodite is entire ; the second pair of 

 gills is always smaller than the anterior pair, the exopodite and sometimes the endopodite 

 is divided by a transverse suture which follows the direction of the suture on the exopodite 

 of the first pair of gills. The terminal appendages of the abdomen or " uropoda " are 

 always comparatively small ; in Serolis cmtarctica they are extremely minute ; these 

 appendages consist of a triangular basal joint attached by the apex to a notch on the 

 lateral margin of the caudal shield, which varies in position in different species, being 

 sometimes at the anterior end of the lateral margin of the caudal shield, sometimes close 

 to its posterior extremity ; the inner angle of the protopodite is always longer than the 

 outer, and the endopodite is longer than the exopodite ; in Serolis latifrons Studer' has 

 correctly described the elongated and spine-like endopodite which becomes completely 

 fused with the protopodite, while the exopodite remain very short ; the appendage is 

 capable of being flexed at right angles to the body, and its sharp spiniform character 

 doubtless serves as a protection against many foes. 



Sexual Characters. — The generative apertures of the male are always situated on the 

 last thoracic segment, on either side of the median ventral line they are always closely 

 approximated, but differ slightly in different species; in some {e.g., 'Serolis antarctica) 

 they are so close together as almost to form a single orifice ; in others {Serolis schythei, 

 &c.) they are a little further aj^art. 



The female generative apertures are more widely separated fi-om each other than the 

 male generative apertures ; they are situated on the antepenultimate thoracic segment, and 

 have the form of an oval slit, while the male generative apertures are invariably circular. 

 The ova are carried about by the female until the young are fully formed ; the 

 immature females have four ovigerous lamellae (see PI. V. fig. 4) developed on the second to 

 the fifth thoracic segments inclusive, which are short, oval in shape, and attached close to 

 the attachment of the thoracic limbs. Studer ^ was the first to point out that these lamellae 

 become much lai'ger in the mature females with eggs, and overlap each other in the 

 middle line, those on the right generally covering those on the left ; this disposition of 

 the ovigerous lamellae, though general, is not, however, universal ; occasionally {e.g., Sei'olis 

 convexa) the arrangement of the lamellae alternates, the right hand lamellae of the second 

 segment overlapping that of the left ; in the two succeeding segments the left hand lamellae 

 overlaps the right, and finally the disposition of the lamellae in the fourth segment is like 

 that in the second. These cases appear, however, to be irregular, and not to be char- 

 acteristic of different species. When these brood lamellae are fully developed the sterna 



1 Archivf. Naturgesch., 1879, loc. at. 2 ^pj_ ^.j-^.^ p. 20. 



