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PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvni. 



The fourth pair of legs are very different from the others, and they 

 vary greatly in the different species and genera, thus furnishing often 

 a useful means of identification (see Plates). In Caligus, Lepeoph- 

 theirus, 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 endopod. 



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. Each exopod joint bears at its outer distal corner a 

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

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

 species there are short semicircular fringed laminae at the bases of 



Fig. 15.— Third swimming leg of adult female Caligus bonito, ventral view. 



these spines on the ventral surface {Caligus rapax, teres, mutabilis, 

 etc.). In Gloiopotes 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 

 find all gradations from the mere rudimentary stumps in the genera 

 Alebion and Pseudocaligus up to legs which reach far beyond the tips 

 of the anal laminae {Caligus longipes, Lepeoplitheirus longipes, Caligus 

 nanus, etc.). In a few species the spines on these legs are widened 

 and flattened into laminae covered with hairs {Caligus hirsutus, C. 

 platytarsi). 



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

 of the genital segment. They consist of a short lamellar basipod, 

 terminated by two or three short spines or setae (5, fig. 1). These 

 fifth legs are usually more prominent in the male than in the female, 

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



