forward with a whiplike motion that draws the ani- 

 mal along. It usually cuts a spiral path in the water 

 but moves in a fairly straight line. 



Now that spindles are no longer so common as 

 they used to be, words like "fish-shaped" or "sub- 

 marine-shaped" are replacing "spindle-shaped" in 

 descriptions of these little animals, which tend to 

 be widest in the middle and tapering toward both 

 ends. The front end may be more rounded than the 

 rear, or just the reverse. In either case the end that 

 goes first in locomotion has some sort of depression 

 into which the flagella (usually one to four in num- 

 ber, but sometimes eight or many) are inserted. 

 The outer layer of most flagellates is firm enough to 

 maintain a constant body shape. The surface cover- 

 ing may be a stiff pellicle handsomely sculptured 

 with spiral or longitudinal ridges, or it may be thin 

 and plastic and allow for squirming "euglenoid 

 movements," named for the familiar fresh-water 

 Euglena in which they are most often seen. A com- 

 pletely constant shape is shown by those dinoflagel- 

 lates that are enclosed in hard skeletons. When a 

 hard surface layer is absent altogether, the animal 

 may be ameboid, at times even losing the flagellum 

 and moving about by extending pseudopods. 



The flagellates are the only group in either plant 

 or animal kingdoms that utilizes all three main meth- 

 ods of feeding: photosynthesis, saprozoism, and in- 

 gestion of solid food. Even a single flagellate species 

 may use the whole repertoire. The photosynthetic 

 flagellates feed like plants, and all have pigmented 

 bodies containing the green pigment chlorophyll; but 

 the green may be masked by additional pigments 

 that make it appear red, yellow, or brown. Even flag- 

 ellates that lack chlorophyll may show, in the proto- 

 plasm, refractive bodies which contain reserves of 

 starch or a similar substance. These have a pale blu- 

 ish green tinge and should be easily distinguishable 

 from the bright green color of chlorophyll. Food re- 

 serves also include oil and fats. Photosynthetic flag- 

 ellates usually have a pigmented eyespot or stigma, 

 near the base of the flagellum, that partly shades a 

 highly light-sensitive region of the protoplasm and 

 enables these animals to orient readily to light and to 

 remain as much as possible in the degree of light in- 

 tensity at which they carry on photosynthesis most 

 advantageously. Parasitic flagellates are entirely sap- 

 rozoic, absorbing dissolved nutrients through the 

 body surface. Most free-living colorless flagellates are 

 animal-like feeders that take in solid food. 



Reproduction is almost entirely asexual in flagel- 

 lates, though some species do show sexual reproduc- 

 tion. In the usual asexual method the body splits 

 lengthwise down the middle, beginning at the front 

 end and proceeding toward the rear. Often this oc- 

 curs at a definite hour of the day. Some always di- 

 vide while in an encysted state; others reproduce 



within cysts only at certain times. Resistant cysts 

 are formed readily if conditions change. Colony for- 

 mation is widespread, especially among green forms, 

 and in such colonies there may be division of labor 

 between ordinary feeding individuals and those that 

 can reproduce, and between reproductive cells that 

 form male or female sex cells. 



Some of the green (zoochlorellae) and the much 

 more common yellow or brown algalike cells (zoo- 

 xanthellae) seen in the bodies of a great variety of 

 protozoans and metazoans, most of them marine, 

 have been shown to be modified flagellates. These 

 live imprisoned within the bodies of their hosts and 

 escape as free-living forms only at certain times. 

 Within the transparent host body they enjoy a place 

 in the sun, yet they are well protected and have all 

 about them a steady supply of carbon dioxide and 

 more especially of other waste products of the host. 

 From these they can obtain the nitrogenous com- 

 pounds that are at such a premium in the tropical or 

 warm waters that are home to most of such flagel- 

 lates. The host receives oxygen and probably bene- 

 fits from the removal of its wastes. Whether it also 

 receives food or uses the pigmented cells as a food 

 reserve is not clear in most cases. 



As a group the flagellates lie somewhere between 

 the algae and the amebas, overlapping somewhat at 

 the edges with both groups. The nature of the over- 

 lap makes it quite plausible that the green flagellates 

 are the ancestral group from which both plant and 

 animal kingdoms have been derived. The ancestral 

 group has remained, on the whole, the most primi- 

 tive of the five classes of protozoans, but particular 

 members are among the most complex protozoans 

 that we know about. There are many different orders 

 of both plantlike and animal-like flagellates, but 

 only some examples can be given, of species most 

 commonly seen or of some interest for the ways in 

 which they benefit or annoy man. 



THE PLANTLIKE FLAGELLATES 



{Subclass Phytomastigina ) 



THE CHRYSOMONADS 



Typical of the chrysomonads ("golden units") is 

 the oval ChroinitUna, with two large pigment bodies 

 in which the golden-brown color masks the green 

 chlorophyll. When it is abundant enough in fresh 

 waters, the water appears brown. Any single indi- 

 vidual, however, is less than ^{^^^uu\ of an inch long. 

 Its one flagellum whips the water, pulling the body 

 along in the fast vibratory glide characteristic of 

 flagellates; but there are times when it uses pseudo- 

 pods to move like an ameba. Dinobryon has two un- 

 equal flagella which protrude from the transparent, 

 vase-shaped cellulose case that encloses the animal. 

 It lives either as a solitary individual or as a branch- 



;] 



