4i6 



A HISTORY OF FISHES 



Nearly all fishes are infested to a greater or lesser extent by 

 animal parasites of various kinds, some of them causing serious 

 discomfort or injury, others apparently harmless. Two kinds 

 may be recognised: (i) those that hve on the external surface 

 of the host (ectoparasites), and (2) those that live inside the 

 host (endoparasites). The first category includes the Leeches 

 and Crustaceans, the second the Protozoa and worms of various 



kinds. Leeches are 

 frequently found at- 

 tached to fresh-water 

 fishes, but, although 

 they suck the blood 

 of their hosts, they 

 do little harm unless 

 present in large 

 numbers. Among the 

 crustacean parasites 

 are included the Sea 

 Lice and Gill Mag- 

 gots. Fresh-run Sal- 

 mon and Sea Trout 

 are generally infested 

 with Sea Lice {Lepeo- 

 phtheirus) , belonging 

 to the group of 

 Crustacea known as 

 Copepods, but diflfer- 

 ing from their free- 

 swimming relatives in 

 having fewer seg- 

 ments and limbs, and 

 in having their organs 

 modified in various 

 ways to fit them for 

 their pecuHar mode of life (Fig. 141). They become firmly 

 attached to the body of the fish, living entirely on the nourishment 

 obtained from its blood. They are unable to Hve for any length 

 of time in fresh water, so that their presence is a sure indication 

 that the fish has recently left the sea. Females are much more 

 numerous than males, and are considerably larger, measuring 

 about three-quarters of an inch in length. Other forms of 

 parasitic Copepods occur in both marine and fresh-water fishes, 

 and many of them are so much modified for a parasitic life 



Fig.141. 



Sea Lice (Lepeophtheirus salmonis) on Salmon. 

 a. Single specimen, X 3. 



