DISEASES AND CAUSES OP DESTRUCTION. Cvii 



many affections from which fishes suffer, from the fungus of the young to 

 the numerous diseases of a contagious or non-contagious character. 



There are certain animal parasites which affect fish, and which may be 

 briefly divided into such as are (a) internal or entozoa* and (b) external or 

 epizoa,) irrespective of which are infusoria and parasitic fungi. 



Entozoa are very common, and it has been computed that each fish may 

 probably have, as an average, not less than' four distinct species of guests 

 able to occupy its body. Tapeworms would seem to be very numerous, 

 but most of their entozoa appear to undergo transformation after changing 

 their abode, the final host being often a water-bird. 



Epizoa are seen as small crustaceans, many having the mouth modified 

 into a suctorial tube or beak, within which are lancet-shaped mandibles 

 employed for piercing. Of these epizoa we have two large subdivisions : 

 first, such as are essentially surface forms, as fish lice, which can move from 

 place to place by means of their hooked and prehensile antennas, or even leave 

 the fish and swim freely in the water ; secondly, the more sedentary forms, 

 as Lernea, having their heads frequently embedded in the bodies of their 

 victims, and without powers of locomotion. Often the whole of their external 

 organs are rudimentary, and they may be found in fishes' eyes, gills, mouth, 

 vent, nostrils, and fins. 



Infusoria are everywhere, in fresh or saline water, and some forms are 

 endoparasitic in the alimentary canals of fishes, as well as other parts of 

 their bodies. 



Parasitic fungi may also occur : thus one of a highly contagious character 

 has been of late years very destructive among fresh-water fishes. This 

 fungus, Saprolegnia ferax (see vol. ii, page 81), has probably been always 

 present, but requires a soil suitable for its germination and growth, and 

 although some of the following may be the predisposing cause to the disease, 

 still it has been observed where none such could have existed. The fish, 

 particularly salmon, may be rendered susceptible from many causes, as 

 debility, and especially after injuries occasioning abrasions, as male kelts 

 after the breeding season, and also unspent kelts, but young fish may 

 likewise be affected. Frosts, droughts, and polluted waters favour its 

 development, and possibly were there fewer kelts preserved, and the waters 

 of our rivers purer, we should find less of this disease, especially where the 

 currents are rapid. Overfeeding appears to predispose to it in some pieces 

 of water. The use of rock-salt is believed to be the best mode of 

 treatment in our fish-ponds and aquaria, while migrating to the sea would 

 seem to arrest the fungus, although it is not certain that it will not reappear 

 on the fishes' return to the river. 



* See Cobbold on the Entozoa : he most correctly observes that cooking fish infested with 

 worms destroys their vitality, while these forms are not capable of existing in the human body. 



