zoans with a full share of the irritability and modi- 

 fiability that are characteristic of all living proto- 

 plasm. Whether this involves "consciousness" is 

 something we can only guess about. A single cell 

 cannot provide the complex sense organs or nervous 

 system that we see in higher animals, but protozo- 

 ans apparently do use flagella, pseudopods, and cilia 

 as tactile organs, and probably also as chemorecep- 

 tors to detect food or chemical changes in the water. 

 Near the front end of many green flagellates there is 

 a specialized photoreceptor in connection with the 

 red-pigmented eyespot or stigma. Many ciliates, in- 

 cluding Paramecium, have been shown to have a 

 neuromotor system, a counterpart, within the cell, of 

 a nervous system, which conducts information from 

 one point to another and which coordinates the beat- 

 ing of the cilia. Nevertheless, we know that percep- 

 tion, conduction, and responsiveness can all occur in 

 what appears to be undifferentiated protoplasm. 

 Protozoans in general probably are sensitive over 

 the entire surface of the cell to such stimuli as light, 

 contact, excesses of heat and cold, concentration of 

 chemicals, or presence of food. And when they re- 

 spond, they usually do so by a movement of the 

 whole animal. The responses of protozoans are ster- 

 eotyped, but no more so than the reflexes of higher 

 animals. And they are not invariable. 



Body Structure 



In touching briefly on the ways in which protozo- 

 ans move about, feed, grow, and reproduce, we are 

 reminded again that they perform all of the same 

 life activities as do the other animals among which 

 they must live and compete. The clear implication 

 is that the "simple protozoans'" are not as simple as 

 they appear to the human eye, even though some of 

 them have little visible structure. There are, more- 

 over, some protozoans that are among the most com- 

 plex cells known. One ciliate, Epidinium ecaudatum, 

 displays at least forty-eight protoplasmic structures 

 that can be described and named. This exceeds the 

 complexity of some of the lower metazoans. The 

 endless variety of protozoan structural specializa- 

 tions can hardly be discussed adequately in any- 

 thing less than a good-sized treatise. Those merely 

 alluded to here are the ones that are visible in the 

 accompanying photographs. 



The nucleus is the one structural specialization or 

 organelle that is consistently present and indispen- 

 sable. It is not always easy to see in the living pro- 

 tozoan, especially in a photograph. In a stained 

 preparation it usually stains much darker than the 

 unspecialized protoplasm or cytoplasm. The nucleus 

 may appear quite different during the various stages 

 of the life cycle. A protozoan that has recently fed 

 contains conspicuous food-filled globules, the food 

 vacuoles. These are not specialized structures but 



merely droplets of water containing ingested food in 

 various stages of digestion. The surrounding proto- 

 plasm secretes digestive enzymes into these food 

 vacuoles, and as the food body undergoes digestion 

 it gradually dissolves, the dissolved substances pass- 

 ing into the protoplasm to be used there for supply- 

 ing energy or growth needs. Flagellates that manu- 

 facture their own food by photosynthesis have no 

 food vacuoles (with rare exception), but instead 

 have one or more prominent pigment bodies, the 

 chromatophores ("color-bearers"). These may be 

 bright green, yellow, green, or brown — depending 

 upon how much yellow or red pigment is present to 

 mask the bright green color of the photosynthetic 

 pigment, chlorophyll. A conspicuous organelle is the 

 contractile vacuole, a pulsating clear globule that ac- 

 cumulates and expels to the exterior excess fluid 

 from the protoplasm of many protozoans, espe- 

 cially fresh-water species. Such vacuoles are usually 

 absent in marine or parasitic forms other than cili- 

 ates. In ciliates the feeding habits tend to increase 

 the amount of fluid taken into the body. There is no 

 very good evidence for supposing that the contractile 

 vacuole also acts as a special device for ridding the 

 organism of metabolic wastes, in the manner of the 

 vertebrate kidney. And in any case this could not be 

 its major role, since so many protozoans are able to 

 do without a contractile vacuole. Usually there is 

 only one such vacuole, as in Amoeba proteus, but 

 Paramecium has two, and some protozoans have 

 many. The locomotor organelles, and an extraordi- 

 nary array of skeletal structures that encase or sup- 

 port the delicate protoplasm, especially of the flagel- 

 late and ameboid types, will be described in connec- 

 tion with the groups in which they occur. 



The Flagellates 



{Class Flagellata or Mastigophora) 



The flagellated protozoans or "whip-bearers" are 

 the most widely distributed of the protozoans, occur- 

 ring in every place that it is moist, from hot springs 

 to the melting surfaces of glaciers. In all seas the 

 greatest portion of the protozoan component of the 

 floating surface population consists of flagellates. Al- 

 most any bit of unlovely green scum from the surface 

 of a pond, when mounted in a drop of water on a 

 microscope slide, will suddenly become transformed 

 into a field of shimmering, green, ovate creatures 

 that swim rapidly but with a jerkiness that distin- 

 guishes them from their more smoothly gliding cili- 

 ated relatives. The jerkiness is not due to an inter- 

 mittent supply of power but to the rotation and gyra- 

 tion of the flagellate body as the flagellum is thrust 



[21 



