sea-fisheries laboratory. 205 



Ninth Day. 

 Fish Parasites. 



Nearly all animals harbour parasites. When one 

 animal lives attached to another, either clinging to the 

 skin,. or living inside the body, and feeding on the blood or 

 juices of the animal to which it is attached, it is called a 

 parasite. 



Thus the fish-louse found on the skin of the flounder 

 is a parasite of this fish, and the flounder is said to be the 

 host of the fish-louse. 



But the pea-crab which lives inside the shell of the 

 mussel is not a parasite, for it does not live on the blood 

 or juices of the mussel. It gets its food from the small 

 animals and plants in the water which enters the shell of 

 the mussel. 



The little white warts which are occasionally found 

 on the skin of the flounder are also parasites, and not the 

 eggs of the fish as many fishermen imagine. 



Barnacle s. — Three different kinds of barnacles 

 are frequently found by fishermen. 



1. The common barnacle, or " scab," which grows on 

 the bottoms of boats during the spring. 



2. The ship barnacle. This does not grow in British 

 Seas, but occasionally comes here attached to floating 

 wreckage. 



3. The crab barnacle. This is a round soft animal 

 which is often found attached to the under side of the tail 

 of different kinds of crabs. It is not the spawn of the 

 crab as many fishermen imagine ; if it is opened its own 

 spawn will be found inside the body. 



The common barnacle and the ship barnacle are not 

 parasites, but the crab barnacle is. It is always a female 

 which is parasitic on the crab, and the male barnacle is 

 parasitic on the female, and lives attached to it. 



These three barnacles are very like each other when 

 they are just hatched. 



