The myonemes of protozoans serve in the main only for alteration of the form of the individual's body. Some protozoans can pull themselves together like a hedge hog or as a snail draws itself into safety within its shell. They do not do this slowly, however, but suck together with lightning rapidity; the best known ex ample is the pulling together of the stalk muscle of the vorticellids. Some proto zoans can creep along with the help of their myonemes in a way probably not very different from snails. The rapidity of all these kinds of movement is very variable. The most efficient are ciliar and flagellar movement, although even these are in no way comparable in their effect with the rapid movements of the higher animals. The movements of flagella and myonemes are often amazingly quick yet the greatest speed they could induce in the protozoan would be usually not more than or even a second. Creeping by muscles or pseudopodia is naturally much slower, at the most 3h 6. in. a second.
Physiology of Sensation.—Protozoa are sensitive to the fol lowing stimuli: (I) light ; (2) mechanical disturbance, such as a blow; (3) change of temperature; (4) chemical stimuli ; (5) electrical stimuli. Not all protozoans react to all of these stimuli; there are, for example, many protozoans that seem to have no sense of light.
Proper sense organs are as yet scarcely known amongst the Protozoa. Only a per idinean (Erythropsis) has a stigma con structed in a complex fashion which may be thought to function like an eye, al though this has not been proved. Prob ably, however, all stigmata serve for light perception and some cilia may sometimes function as feelers.
The mode of transmission of sensations, like the presence of sense-organs, is as yet unknown. When the fore-end of certain protozoa is disturbed, and as a result the whole animal contracts, we can only con clude that the point touched has given an impulse to the myoneme.
Practically the only reactions we can recognize at sight in protozoans are those consisting in movement. Light-sensitive protozoans swim towards the light. Touch sensitive protozoans contract when they are touched, alter their direction of movement or try to swallow the offending object.
Some chemical stimuli, e.g., weak acids, have the effect of frighten ing protozoans away from the region, whereas others such as oxygen or a food-particle have the opposite effect.
The method of reproduction which is found most frequently is simple division into two. An individual splits into two pieces of which each soon appears just like the animal before splitting (fig. 7, 8). This seemingly simple division is in reality a very complex process. First the nucleus divides and if several are present they all divide about the same time; then the cell body usually splits by stretching out and being constricted; a ring shaped furrow gradually cleaves the cell in two as if a loop of string had been put round the middle and were being pulled tight. This division furrow has usually a defined position on the axis of the protozoan; some divide longitudinally (fig. 7), others trans versely (fig. 8).
In many cases the "new-born" daughter-animals, as mentioned above, are soon indistinguishable from their parents ; this means that they have reconstituted the set of cell-organoids, which is done for some of the latter, such as the chromatophores, by divi sion as in the nucleus ; other organoids are merely distributed and each daughter-cell makes good those it lacks; in this way the fla gella and cilia are divided. When the basal granule splits one piece of it takes the flagellum with it and the other forms a new flagellum (fig. 7). Finally there are organoids in no way taken over from the old individual, having been pushed out during the division or even dissolved in the protoplasm. Such organoids are formed anew in each daughter animal (fig. 8). Protozoa that are enclosed in a hard shell often divide in such a way that although the protoplasm has divided quite equally only one of the two parts takes over the shell while the other slips out and builds a new shell.
Nuclear Division.—Before considering the other kinds of division, something must be said of the division of the nucleus in the Protozoa. In multicellular plants and animals division is a highly developed process whose most important feature is the ap pearance of thread-like or rod-shaped bodies called chromosomes.
At the same time—often under the influence of the centrosomes, ball-shaped organoids present in the cell at first singly but at division splitting into two separate halves—a fibrous, often spindle-shaped body is constituted; this is the spindle, and to it the chromosomes attach themselves in such a way that they come to lie in a plate across the spindle, at either end of which the centrosomes are found.