FISHES AND MANKIND 



417 



that their crustacean affinities are scarcely recognisable, and 

 it is only when their whole life history is known that their place 

 in the system can be determined. Generally, the conspicuous 

 egg-sacs draw attention to the presence of the parasite, whose 

 head and anterior parts may be buried in the tissues of the 

 host. 



The so-called "maggots" [Salmincola) infesting the gills of 

 Salmon and other fish are more remarkable, and are much less 

 crustacean-like in appearance. One pair of limbs has been 

 modified into long arms, uniting at the tip to form a sucker, 

 by means of which the animal adheres to the gills. At the 



Fig. 142. 



Gill Maggots {Salmincola salmonea) on gills of Salmon. 

 a. Single specimen, X3. 



Other end of the shapeless, fleshy body is a pair of long egg-sacs, 

 each containing several series of eggs (Fig. 142). The males of 

 this genus are little known, but are dwarfed, and sometimes to 

 be found attached to the females, which grow to a length of a 

 little more than a quarter of an inch. 



Mention may also be made of the so-called Carp Louse 

 {Argulus) , another crustacean parasite resembhng the Copepods, 

 although its relationship to that group is now considered 

 doubtful. This creature has a broad, flat, and very transparent 

 body, about three-sixteenths of an inch in length, and attaches 

 itself to different species of fresh-water fishes by means of a 

 pair of large round suckers on the under side of the head. It is 

 only a temporary parasite, and is frequently found swimming 

 free in ponds and rivers. It sucks the blood of the fish, but 



