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



the upper side, transversely concave on the lower, and longitudinally flattened in 

 correspondence with the surface of the preceding joint. These joints are fringed with 

 hairs that vary in different species, but as a whole they are of little importance except 

 as carriers of the terminal flagella. 



These flagella are always two in number. In some genera they are of equal length, 

 while in others they are unequal ; they are evidently of different degrees of importance, 

 as one is liable to vary with the sex, and is generally furnished with peculiar organs that 

 are evidently connected with some special sense. 1 Though one flagellum may in 

 different species vary in length it never becomes rudimentary, as the diminution is clue to 

 the shortness, rather than to the numerical decrease, of the articuli, whereas the second is 

 always slender and constructed of articuli that are long, and the diminution generally 

 takes place by their numerical reduction. 



In an adult specimen of Penseus japonicus, where both flagella are short and of 

 equal length, neither being longer than 5 mm., the primary flagellum consists of fifty 

 articuli, and the secondary only of twenty. In Aristeus, the primary is very short and 

 the secondary very long ; the former is flattened and hollowed on the lower side, which 

 latter character is emphasised in Solenocera to such a degree that the more slender 

 flagellum when at rest is lodged within the longitudinal hollow of the larger. 



The second pair of antennas articulates freely with the metope, and consists of a 

 peduncle of five joints and a long flagellum. The first joint is generally short and 

 broad, and carries on the inner side a large phymacerite, at the extremity of which 

 is a passage closed by a soft membrane. This is the external passage connected with 

 the green gland, the function of wiiieh has not yet been determined. The second joint 

 is longer but not so broad, and supports at its extremity a scaphocerite, which in this 

 tribe of Crustacea is large, being broad, thin, and foliaceous, and on an average about 

 one-fourth the length of the entire animal. The outer margin is strengthened by a 

 longitudinal rib that terminates in a sharp tooth more or less distant from the distal 

 extremity. The form varies in different genera. In some it is long and broad as in 

 Aristeus ; in Sergestes it is long and narrow, and in Sicyonia it is broad at the base, and 

 gradually but obviously narrows from its greatest diameter to the apex. The scaphocerite 

 is strengthened at the outer margin, sometimes by one, but in others, as in Sicyonia, by 

 two longitudinal ribs that converge towards the extremity, where they unite and form 

 the external distal tooth ; from the inner or median rib a series of parallel ribs or raised 

 lines run obliquely to the margin, and, when they approach it, widen and divide into 

 two or three others. In Penzeus the median longitudinal rib does not converge towards 

 the subapical tooth, but runs down the centre and fades away before it reaches the distal 



1 These organs I believe to be endowed with acoustic properties, but Clans suggests that they may be olfactory organs. 

 They are not so constant in the Peiia'idea as among other Crustacea, where, when present, they exist as translucent 

 membranous cilia of variable form. 



