FRESHWATER LIFE INFUSORL\. 289 



FEESHWATEE LIFE.— in. INFUSOEIA. 



BY EDWIN SMITH, M.A. 



If a little hay is steeped in watei- for a few days, and the infnsion is 

 then examined, it will be found to teem with microscopic life. Sirailarly 

 the natui'al infusions offered by accumulations of water containing 

 decaying animal and vegetable matter, or ponds where the simpler forms 

 of vegetation flourish, are tenanted by countless millions of minute 

 animals of various kinds, which, from their mode of occurrence, were 

 named by the earhest observers Infusoria. The term at first included 

 many organisms which, further investigation showed, could not be 

 retained in the same class ; plants mistaken for animals, because they 

 moved about ; animals of higher organisation, such as the Eotifera ; 

 others of a lower type, like the Amoeba. This sifting process is even yet 

 far from complete. It is not improbable that many forms now placed in 

 this class, when their hfe histoiy comes to be written, wiU have to be 

 separated from the Infusoria properly so called. The number of species 

 may also be reduced. For in the imperfection of our knowledge it is 

 well to remember that forms which to-day are counted as distinct 

 species may hereafter prove to be only different stages of the same 

 animal. 



Like other members of the sub-kingdom Protozoa, the creatures we 

 are considering possess a simple body not divided into segments, and one 

 which cannot be cut into two exactly coiTesponding halves. There is no 

 definite ahmentary canal, but digestion is effected indifferently in any 

 portion of the fluid contents of the body. Pellets of food may be lodged 

 in vacuoles extemporised in various parts of the interior, but they are 

 not enclosed in stomachs separated by any sort of wall from the 

 surrounding mass. Compared with other Protozoa, the Infusoria exhibit 

 a more advanced difterentiation of structure. The fluid protoplasm or 

 sarcode, of which the bulk of the body consists, passes externally into 

 a denser portion, the so-called cortical layer ; which again is often 

 protected by a still firmer covering termed the cuticle. Food is admitted 

 by a distinct ciHated mouth opening into a short ciliated gullet, whence 

 it passes, together with a small quantity of water, into the general body- 

 cavity. When a proper mouth is not present, there is at least an oral 

 region where a mouth may be extemporised. Refuse is excreted at a 

 particular spot situated near to or remote from the mouth ; but the 

 discharging orifice is not, as a rule, permanently visible. One, two, or 

 more contractile vesicles, having a fixed position in the cortical layer and 

 connected with channels leading inwards, serve by then- slow expansion 

 and quick contraction to keep up a sort of cii-culation in the fluid interior, 

 and to purify the sarcodic contents. In the same layer are found a 

 nucleus, the female element, and, attached thereto, a nucleolus, the male 

 element of the reproductive process. Reproduction takes place either by 

 self-division lengthwise or cross-w^se, or by the conjugation of two 

 individuals ; the former method being characteristic of the sedentary, the 

 latter of the free-swimming Infusox-ia. The same species may multiply 

 in both ways. 



