THE INFUSORIA 461 



ably owing to the fact that under natural conditions only a very small propor- 

 tion of the young parasites succeed in establishing themselves on a fish, and 

 consequently the infections produced are so slight that they are overlooked, 

 and the fish is unharmed. 



The life-cycle of IcMhyophthirius is as follows : The youngest parasites 

 hatched out from a cyst are very small, and have a macronucleus and a micro- 

 nucleus. They seek out a fish and bore into its epidermis, attaching them- 

 selves by one end of the club-shaped body and rotating actively, with the 

 result that epithelial cells are displaced, fcnd either cast off into the water or 

 form a ring-like wall round the parasite. In this way the infusorian works 

 its way gradually into the deeper layers of the epidermis, which closes over it, 

 so that the parasite lies finally in a closed hollow space in the epidermis. In 

 this position it grows in size, and at a certain point the micronucleus disappears, 

 passing into the macronucleus to form a nucleolus-like body within it. The 

 parasite appears to the naked eye as a little white spot on the skin, occurring 

 on any part of the body-surface or on the gills. It retains its cilia, and can be 

 seen rotating within the cavity in which it lies. 



The full-grown Ichthyophihiriua may reach 1 millimetre m diameter, but is 

 usually less, about 0'75 millimetre. When full-grown the ciliate breaks out 

 of the cavity in the epidermis and sinks to the bottom, attaching itself to .the 

 ground or to water-weeds, and becomes encysted. Within the cyst it multi- 

 plies by binary fission repeated eight times, producing 256 small ciliates ; 

 sometimes this multiplication takes place without encystment. During this 

 process of multiplication the micronucleus reappears, being extruded from 

 the macronucleus of each individual when not less than four are present in 

 the cyst ; but the exact period at which the micronuclei appear varies in 

 different cases. In addition to the micronucleus, one or two other extrusions 

 from the macronucleus take place (Buschkiei) ; but whether these represent 

 other micronuclei or expelled vegetative chromatin is not clear ; in any case 

 they degenerate and disappear. When the micronucleus makes its appearance, 

 it divides by mitosis at each division of the cell-body, as in ordinary Ciliata, 

 while the macronucleus divides in the usual way by direct division. 



When the full number of tiny ciliates is formed, each with a macro- 

 nucleus and micronucleus, sexual phenomena occur, but the events that 

 take place are described differently by different investigators. According to 

 Neresheimer (858), in each individual the micronucleus divides twice, and 

 three of the four micronuclei produced degenerate ; the fourth then divides 

 again. The reduction-process is, therefore, according to this account, similar 

 to that of other Ciliata, and the organism appears to be ready for con jugation, 

 with two pronuclei ; but Neresheimer was unable to observe conjugation taking 

 place either in the cyst or after the organisms have become free ; he observed, 

 however, sometimes two micronuclei, sometimes one, both in free forms and 

 in those attached to the fish, and from this it was inferred that the two pro- 

 nuclei fuse autogamously, leaving the possibility open, however, that heter- 

 ogamous conjugation might sometimes occur. According to Buschkiei, on 

 the other hand, the micronucleus of each individual divides twice, and, of the 

 four thus produced, two degenerate, and the remaining two fuse autoga- 

 mously while still within the cyst. 



The little ciliates are set free from the cyst, and seek out a new host in their 

 turn. / From the time that the full-grown parasite leaves the fish to the time 

 that the brood is liberated from the cyst is, according to Buschkiei, about 

 twenty hours, more or less. If an infected aquarium be kept empty of fish 

 for sixty hours, it becomes disinfected, since the parasites all die off if they 

 cannot attach themselves to a fish very soon after they are hatched out. 



The entozoic Ciliata, in which adaptation to a purely parasitic life has led 

 to the degeneration of the apparatus of a holozoic mode of nutrition that is 

 to say, of the mouth, peristome, and accessory cilia are sometimes classified 

 as an order, Astomata, of the Holotricha ; but there can be little doubt this 

 group, like others founded on negative characters, is a'heterogeneous collection 

 of forma in which the characters they possess in common are due to convergent 



