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PROCEEDINGS OF THE NATIONAL MUSEUBI. 



VOL. 39. 



are much more widely distributed than the Cahgid^. The latter 

 family contains three or four times as many species as the former, yet 

 only one or two of them are cosmopolitan, while in the Ergasilidae 

 there are half a dozen or more found in all parts of the world. 



Food. — Living as they thus do upon the fish's gills, there can be 

 but little doubt that they feed upon blood. 



Such a conclusion is further evidenced by the structure of the 

 mouth-parts, which are so degenerate as to be unfit for biting or chew- 

 ing but are well suited for piercing such 

 delicate tissues as cover the gill filaments. 



MORPHOLOGY. 



General body form. — As in the CaligidsB 

 the body of an Ergasilid is made up of 

 four parts or regions, a cephalon or ceph- 

 alothorax, a free thorax, a genital seg- 

 ment, and an abdomen (fig. 1). 



The first thorax segment is generally 

 united with the head to form the cepha- 

 lothorax, the two being covered with a 

 carapace which in many species is so 

 strongly inflated that it overlaps the fol- 

 lowing thorax segments to a greater or 

 less degree. In the females of the Bo- 

 molochinse and Tseniacanthin^e and in the 

 genus Thersitina the fusion is complete, 

 and there is no line of demarcation visible 

 between the two. But in the females of 

 the genus Ergasilus the fusion is not com- 

 plete, and there is a well-defined groove 

 or at least a pair of notches in the lateral 

 margins of the carapace to indicate the 

 point of union. In the males of nearly 

 all the genera the first thorax segment is 

 free like the others. This cephalothorax 

 is more strongly arched than in the Cali- 

 gidie, and in the genus Thersitina it be- 

 comes almost hemispherical. This is at 

 least partly explained by the fact that in the females the ovaries and 

 ovarian diverticula are just beneath the carapace and require consider- 

 able space, especially when the eggs are fully developed. The cara- 

 pace is perfectly plain and without sinuses ; the only grooves visible 

 are a horseshoe-shaped groove, which in some species surrounds the 

 cephalon proper, very similar to that in the Argulidse, and in the 

 genus Ergasilus a transverse groove separating from the rest of the 





Fig. 1.— Side view of a female Er- 

 gasilus MANICATUS, SHOWING BODY 

 regions: ^.Abdomen; C T .Cepha- 

 lothorax; E. C, Egg cases; F. T., 

 Free thorax; G. iS., Genital seg- 

 ment. 



