no: 1404. PARASITIC COPEPODS—CALIGIDJE— WILSON. 571 



that of the adult, except that the pigment spots are much fewer in 

 number and more widely separated. Length 2 mm., increasing- grad- 

 ually to 8.5 or 4 mm. Width of carapace 1 mm., increasing to 2 mm. 

 Width of free segment0.55 mm. , length 0.55 mm. Length of abdomen 

 0.8 mm. 



(rapax, rapacious, greedy.) 



This is the most common species of the genus on the northeastern 

 coast of the United States, having been taken from more than twenty- 

 five different kinds of fish by many collectors working in the interests 

 of the United States Fish Commission. 



The author, following the example of Mr. Richard Rathbun, as 

 expressed in some very valuable manuscript notes, the substance of 

 which is here presented, has been liberal in his interpretation of specific 

 characters. In consequence, there are included under this species indi- 

 viduals which some investigators would probably subdivide into several 

 lots. But after a very careful comparison the differences observed are 

 reall} T too obscure to be of much value for classification. The variation 

 does not extend far in any one direction, and there is comparatively 

 little difference in size. The detail of the appendages agrees very 

 closely in all the specimens, and careful comparisons have been made 

 in every instance with authentic specimens from Europe. Specimens 

 from the mackerel present perhaps the most marked differences in 

 structure, but they vary among themselves and many are perfectly 

 normal. These differences concern chiefly the fourth pair of legs. 

 Sometimes the second of the terminal spines, counting from the proxi- 

 mal end of the terminal joint, is wanting. Again, the ultimate and 

 penultimate joints are relatively shorter and stouter than in normal 

 specimens; or the three successive spines on the outer margin may 

 overlap one another; or the fringed disks at the bases of these spines 

 may be so little developed as to be overlooked. In specimens from 

 other fish the furca may have shorter and stouter branches, while on 

 the common flounder individuals may be found with the abdomen so 

 much shortened as to appear like those of O. curtus. But in all these 

 instances the other appendages are perfectly normal, and it does not 

 seem as i f these differences were worthy of creating even a well-marked 

 variety, to say nothing of a distinct species. 



They must be mentioned, however, to show that they have been 

 noted and given the proper attention. 



Both sexes are usually found upon the same tish and occur anywhere 

 14)011 the external surface, often showinga preference for the pectoral 

 tins. Upon the flounders and skates they are commonly on the upper 

 or pigmented surface, but also occur on the unpigmented ventral sur- 

 face. Elere again, as in the case of the Arguli, when we reflect that 

 these fish frequent the bottom, and that they often bury themselves 

 in the sand or mud, we can appreciate better the effectiveness of a 



