xxiv INTRODUCTION. 



Fish, like other creatures, are subject to diseases. Various parasites, either in the form 

 of E7itozoa or Epizoa, find a lodgment within or upon their bodies. The internal parasitic 

 hosts are very numerous ; they occur generally in the stomach, pyloric appendages, and 

 intestine, in the form of various kinds of tapeworm {tmiia), or small filamentous annelids (ascarii). 

 Sometimes intermuscular parasites are found, but this is less common, I believe, in fish 

 belonging to this country than in some foreign species. The Epizoa are found on the surface 

 of the bodies of fishes ; various forms of Crustacea, such as Leniaa, LepcopJitharies, Argulus 

 foliaceus, attack them ; the curious little Gyrodadylm clegans I have found inside the gills of 

 the Three-spined Stickleback; the young fry of the fresh-water swan mussel {Anodonta cyg)iea), 

 I have detected on the fins of the Perch, Stickleback, and a few other fishes ; Pike are 

 sometimes, when in a weak and unhealthy state, covered with that curious little discophorous 

 annelid, Piscicola. As a rule, however, I do not think that parasitic guests affect fish very 

 seriously. When epizoa, or external parasites, abound to a great extent, the fish is, no 

 doubt, injured by them, and a weakly condition of the fish may be to some extent the cause 

 of their attacks. But although the presence of animal parasites, whether external on the 

 skin or internal within the viscera, may be unproductive of much serious mischief, it is quite 

 different with certain subtle forms of vegetable growth, which often occasion fearful destruction, 

 like some epidemic amongst the higher animals. 



One essential condition for health in a fish is undoubtedly pure water; this is specially 

 the case in the Sahnonida. Some fish, however, will certainly exist, and apparently thrive, in 

 water which can by no means be called pure. I have taken the Three-spined Stickleback 

 from very foul ditches indeed. Carp and Tench will do fairly well in muddy pools, but they 

 will do better still, and prove more fitted for the table, if kept in ponds supplied by 

 bubbling springs, and containing aquatic plants of various kinds, with a muddy bottom in 

 which they can hybernate In the cold winter. As a rule salt-water fishes are much less 

 liable to suffer from parasitic attacks than fresh-water species. Mr. Jackson, the able Curator 

 of the Southport Aquarium, tells me that fresh-water fish do not thrive In confinement ; 

 that they are extremely liable to be attacked, to an Injurious extent, by parasites, but that this Is 

 not the case with fish in the salt-water tanks. One of the most dreaded and fatal of all the 

 diseases to which piscine nature Is subject, occurs In the form of white flocculent patches on 

 the tall, head, or other parts of a fish's body. Everyone, with the slightest experience, must 

 have observed what I am alluding to. If he has merely kept Goldfish In glass globes. 

 This white filamentous growth is either a fungus, or some plant related to a fungus ; it is 

 known by the name of Saprolegnia ferax, and sometimes does incalculable mischief In some 

 of our Salmon rivers. In ponds or tanks where young Trout are artificially cultivated, 

 thousands often die from the attacks of Saprolegnia ferax ; and I feel certain, from what I 

 have noticed myself, that this fatality is primarily caused by overcrowding In a water which 

 has too high a temperature. It Is generally easier to prevent the appearance of a disease 

 than to stop its ravages when it has once begun. Now it Is an ascertained fact that 

 fungi of all kinds require for their development a certain warmth of temperature, and that 

 cold will prevent the spores of a fungus from germinating. This holds good with regard 

 to the larger kinds of fungi, as well as to those almost infinitesimal atoms, such as 

 Bacteria, Bacillaria, &c., which are doubtless at the root of many zymotic diseases affecting 

 men and other highly organized animals. 



