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



furnished with a median spine. On PI. I. are figures of a female (fig. 2), a fully 

 developed male (fig. 1), and a young male (fig. 3) of Serolis cormtta, which are drawn 

 exactly of the natural size ; and judging from the very slight difference in length between 

 the immature and the fully developed male, it would appear that the male does not 

 acquire all its secondary sexual characters until the last moult, though the comparatively 

 oreater breadth of the body, which also distinguishes the males of this species from the 

 females, is recognisable somewhat earlier. 



The comparatively small number of S2:>ecimeus (seven) of Serolis necera makes it 

 impossible to state with any accuracy the time at which the males assume their proper 

 sexual characters ; the facts are as follows — of the fjur male species two are completely 

 adult, measuring 42 mm. in length, the remaining two are immature, the largest 

 measuring 25 mm. in length ; this specimen has the general appearance of a female in its 

 comparatively short epimera. absence of modification in the third thoracic appendage, and 

 in all the other secondary characters by w^hich the males of this species difi'er from the 

 female, with the exception of the frontal "sense organ," which resembles that found in the 

 adult males, and is not greatly developed as in the females ; from this it appears that 

 those secondary sexual characters in which the males of this in common with other 

 species difi'er from the female appear comparatively late, while the one sexual character 

 peculiar to the species is developed early. 



In Serolis schythei the males reach maturity much sooner than in Serolis corniifa ; 

 the largest male specimen in the Challenger collection measures 30 mm. in length by 

 38 mm. in breadth ; the smallest male specimen, with fully developed secondary sexual 

 characters, is 16 mm. long by 19 mm. broad ; another male specimen, in which the 

 sex could only be detected by the position of the generative apertures and by the short 

 penial filament, measures 15 mm. in length by 17 mm. in breadth; in this species, therefore, 

 the secondary sexual characters which distinguish the male appear comparatively early, 

 and in immature males, which in other respects are outwardly like the females, the greater 

 proportionate breadth is recognisable ; this character is the first to appear, as also 

 apparently in Serolis cornuta. 



Serolis hromleyana. — In this species the males difi'er from the females in the 

 third thoracic and second abdominal appendages, and also in the epimera, which are 

 shorter in the female and slope downwards at a less angle with the longitudinal axis 

 of the body. The young males, as in other species, are closely similar to the 

 females, but acquire the secondary sexual characters peculiar to the males at a 

 comparatively early period ; the largest male specimen of Serolis hroTiileyana measures 

 54 mm. in length, the smallest male, which shows all the secondary characters peculiar to 

 its sex, measures 25 mm. in length; the largest male specimen, which has not yet acquired 

 its proper secondary sexual characters, is 21 mm. in length ; another specimen 22 mm. in 

 length is nearly adult ; the penultimate joint of the third thoracic appendage is swollen 



