508 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXVIII. 



The fourth pair of log.s are very different from the others, and they 

 A'ary oroatly in the different species and genera, thus furnishing often 

 a useful means of identiffration (see Plates). In (jdhjus., Lepe<»j>1i- 

 tJirh-iis^ and allied genera they consist of a basipod usually as long as 

 all the remaining joints put together, and a one, two, or three jointed 

 exopod with no trace of an endo]:)od. 



The exopod is really always three-jointed, but in development the 

 joints often become so thoroughly fused that all trace of the original 

 division is lost. P^ach exopod joint bears at its outer dismal corner a 

 sharp spine, usually straight, and the terminal joint ends in three 

 spines of unequal length, the iiuKM* one being the longest. In several 

 species there are short semicircular fringed lamina' at the bases of 



Fig. 15.— Third swimmin'g leg of adult female Caligts bonito, ventral view. 



these spines on the ventral surface {CaJIgnx n(j)<',(\ f< rcs^ nnitahihs^ 

 etc.). In GJoiopotcH there is a continuous fringe of short, stiff' spines 

 along each joint, on its outer margin. 



The size and length of these legs is even more variable, and we can 

 ffnd all gradations from the mere rudimentary stumps in the genera 

 AJthlon and Pt^eadocallgu^ up to legs which reach far beyond the tips 

 of the anal laminte {(■aUgnK Jongipes^ Lepeophtheh'nx J(ing!j>es^ CaJigns 

 iKiniis^ etc.). In a few species the spines on these legs are widened 

 and flattened into lamintp covered with hairs {Caligiis Jui'ioifus. ('. 

 platytarsf). 



The fifth legs are rudimentary and are attached to the posterior end 

 of the genital segment. They consist of a short lamellar l)asipod, 

 tei'minated by two or three short spines or seta^ (5, fig. 1). These 

 fifth legs are usually moi'C prominent in the male than in the female, 

 where they are often reduced so umch as to be practically lost. 



