226 Desa^iption of a Species of Caligus. 



When disturbed, they move with rapidity over the fish, and either 

 backward or forward with nearly equal facility. In swimming, 

 their motion is equally rapid. They thus travel over the body of 

 the fish at will, and, we doubt not, occasionally leave one fish for 

 another. 



Both sexes frequently occur on the same fish, though the fe- 

 males (during the months of November, December, and January) 

 have been far the most abundant. The latter are, in general, 

 readily distinguished by the two filiform appendages to the body, 

 which are the external ovarian tubes ; or, if these are wanting — 

 as often happens — by the larger abdomen, whose greater size is 

 owing to the eggs it contains. If destitute of eggs, it does not 

 present this peculiarity. The sexes differ, moreover, in the form 

 of the first and third pair of feet, as will be particularly noticed 

 when speaking of these members. 



The sizes of the individuals which have come under our no- 

 tice, have been very various. The adult males frequently attain 

 a length of five ninths or nearly two thirds of an inch, (fig. 2, 

 PL III.) The females are always smaller than the males, and sel- 

 dom exceed a half inch, exclusive of the ovarian tubes, (fig. 3.) 

 The smallest individual seen, was one tenth of an inch long. Its 

 legs had less slender proportions than usual ; otherwise, it did not 

 difi"er from the adults. 



The Caligi live several hours on the body of the cod taken from 

 the water ; but generally die soon after the death of the fish. 

 When taken from the fish and confined, they exhibit a strong in- 

 clination to leave the xvater. We have often observed, after the 

 introduction of fifteen or twenty into a glass of salt water, that 

 the greater portion of them seek the surface, where they attach 

 themselves to the glass ; and quite a number leave the water en- 

 tirely, crawling up the glass an inch or two above the surface. 

 The water they confine under their broad shell, which is closely 

 attached at its margin, supports them for a while ; but, unless as- 

 sisted again to their element, they remain, without any apparent 

 attempt to return, and in a few hours die. 



These animals, like the cod, on which they live, require a low 

 temperature, and have been observed to swim, with scarcely di- 

 minished activity, in water that was freezing. In some instan- 

 ces, when the water had evidently reached a temperature below 

 32° F., without congelation, they have been rendered torpid, and 



