510 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



or vessels scnipiilouslY clean, and especially keep away all decaying 

 animal matter. 



The liigher alg'?e and aquatic plants of every kind may prove hurtful 

 to the fish by limiting the extent of water and hindering the free move- 

 ment of the fish. This is also the case with the so-called "water plague," 

 Wasserpesf, Ulodea canadensis, which, however, does not deserve this 

 name, as it contains much food and develops a great deal of oxygen. 



Among the protozoans which form the connecting link between veg- 

 etable and animal life we find a small but very dangerous enemy of the 

 fish, namely, the Fsorospermia. These are round or oval bodies, often 

 possessing a tail, with an internal cellular formation, measuring about 

 0.005'", which were first discovered in 1841 by Johannes 3fuller in the 

 socket of the eye of the pike and in small pimples on the skin of the 

 perch, the stickle-back, and several fish of the cyprinoid family. They 

 form the contents of small capsules, measuring -J- to h'", which are im- 

 bedded in the membranes. They have also been found in the bladder 

 of pike and codfish. Eecent investigations have thrown some light on 

 the origin of these beings. They are products by separation of the so- 

 called " Gregarines," which develop an amoeba without any kernel, 

 which again changes to a gregarine. lAehcrhvlin has observed the de- 

 velopment of the psorosperm into an amoeba, and E. van Beneden the 

 change from the amoeba to the " gregarine," the one in the pike and the 

 other in the lobster. So far it has not been ascertained with absolute 

 certainty in what way the fish are affected by them. 



It is certain that these parasites occasion the destruction of the tissue 

 which surrounds them by producing festering sores, and that fish in- 

 fected by them must gradually die. 



Certain formations of a higher group — the infusoria — have also recently 

 l)een accused of being enemies of fish ; some of them, such as the OpaUna 

 ranarum.; in the intestinal tube of the frog, and the Trichodina pediculm, 

 have long been known as internal and external parasites. They can 

 get in under the outer skin and destroy it. Livingston Stone recom- 

 mends the transfer for a short time of fish affected in this way to salt 

 water. It is said that among the moUusks the Tioliagonia pohjmorpha 

 does not disdain the spawn of fish. 



We must now turn to a group of animals which has a very bad repu- 

 tation, and which, belonging to the worms, are comprised in the family 

 of intestinal worms or helminths. This group sends a whole army of 

 animals into the field, some only visible tlirough the microscope, others 

 measuring inches and even yards, and often possessing terrible weapons; 

 they live and find their food in the abdominal cavity, the intestines, 

 muscles, gills, and skin of fish. 



The eel contains no less than 25 different kinds of these parasites, the 

 perch 23, the pike 21, the salmon 16, the trout 15, and the carp 12. 



The intestinal worms are divided into four classes, viz, Cestodes, 

 Trematodes, Acantliocepliala, and Nematodes. 



