NOTES ON ICHTHYOLOGY. 31 



as in the latter theory, the result of a morbid condition in the fish, 

 arising from the artificial incubation. 



Certain, however, it is that the "gill fever" is at present as much 

 without a remedy as the cattle plague, and the best chance the fish cul- 

 turist has of saving the healthy fish is by isolation and cleanliness of 

 the strictest kind. 



Another remarkable form of vegetable parasite causing deterioration in 

 the health of fishes has the faculty of boring through hard tissues, such 

 as shells, scales of fish, as well as the hard tissues of animals. I have 

 seen examples of this in the scales of the barbel and old carp. The boring 

 resembles that of the teredo into the timbers of ships which have traversed 

 the tropical seas simple perforations, sometimes in various directions, but 

 always so exceedingly minute as to give the idea of the existence of 

 pores in the scale. Occasionally these perforations resemble nothing BO 

 nearly as the root of a Scotch fir, with its eccentric ramifications. 



Members of the groups included under the term protozoa by Siebold, 

 involving the line of demarcation between animal and vegetable life, are 

 very plentiful on fish. It is obviously impossible, however, to go 

 exhaustively into the subject of their histology. 



Of the better defined animal parasites, huge numbers exist which 

 appear to, in some sort, bear relation to the infusoria and rotifera fami- 

 lies. But, to the exclusion of a consideration of these, I must pass on to a 

 most remarkable little creature which usually attaches itself to a stickle- 

 bat, minnow, or other small fish, and often causes considerable irritation. 

 This is no other than the embryo of the Anoden cygneus, or fresh-water 

 mussel. It has been estimated as a parasite proper, but really is not so ; 

 for a careful observation will reveal to the examiner the various changes of 

 the larva, ending in its final attainment of the bivalve form. On the 

 embryo being ejected from the valves of the parent, it may be found 

 provided with a curious apparatus for " holding on." This consists 

 in a number of barbed serrated hooks, situated like teeth between the 

 valves, which the little animal snaps together continually until firmly 

 embedded in the skin of its fishy "host." I have had minnows with 

 the tail and gills covered with numbers of these creatures, resembling 

 tiny onion seeds. A full description of this very interesting parasite will 

 be found in the "Quarterly Journal of Microscopical Science," N.S., vol. 

 2, 1861, page 162. 



The whole tribe of suctorial Crustacea seem more or less to reside on 

 fish. They imbibe the juices of the " host " by means of a peculiar pro- 

 boscis, which in them takes the place of the jaws of other crustaceans, and 

 thus to a certain extent saps the vital energy. Amongst these -the very 

 common "fish louse" (Argalus foliaceus) takes its place. This insect 



