spines. One or more eyes may be seen on the front 

 end as red flecks. 



From the mouth the digestive tube leads promptly 

 to a gizzard-like swelling, the mastax, which has 

 powerfully muscled hard jaws. Through the trans- 

 parent wall the toothed jaws of the mastax can be 

 seen grinding away at the food that is swept down 

 the mouth. In some species the jaws are long and 

 slender, forming a kind of forceps that can be ex- 

 tended through the mouth to grasp prey. 



The tiniest rotifers are not nearly as small as the 

 smallest protozoans, but members of both groups are 

 generally of comparable size, and the largest rotifers 

 are only about \-,i, of an inch long, not as long as the 

 giant fresh-water protozoan, Spirostomitm. Little 

 wonder, then, that the early microscopists confused 

 the many-celled rotifers with ciliated protozoans, for 

 the two groups are similar in many superficial ways 

 and resemble each other even more in habits. Like 

 ciliated protozoans, the rotifers swim in spiral fash- 

 ion, attach to vegetation and feed by ciliary currents, 

 often live in cases attached to water plants, and have 

 a cosmopolitan distribution. Geography means noth- 

 ing to animals so small that they can be swept along 

 in the feeblest movements of water — and so resistant 

 to drying, either as dormant eggs or as desiccated 

 adults, that they can be carried about by winds and 

 on the feet of birds. After months or even years in 

 the inert state, some rotifers can again spring into 

 activity at the first wetting. If conditions are the 

 same, a lake in Germany, or for that matter one in 

 China or in South Africa, will have the same species 

 of rotifers as one in the United States. Relatively few 

 species are restricted to special conditions, as are 

 those found only in highly alkaline lakes in the west- 

 ern United States, or those that live attached to par- 

 ticular species of aquatic plants. 



Basically, however, protozoans and rotifers are 

 very different, for the latter are composed of the 

 equivalent of a large number of extremely small 

 cells. The cell membranes present in the embryo 

 mostly disappear in the adult, leaving tissues that 

 are protoplasmic masses with numbers of nuclei. 

 Each of the nuclei occupies a definite position, and 

 through the transparent body wall they can be seen 

 and counted. The number of cells of a late embryo, 

 or the number of nuclei of an adult, is constant for 

 any species — usually between nine hundred and one 

 thousand. In their cell or nuclear constancy rotifers 

 are almost unique among multicellular animals, 

 though this phenomenon does occur to a lesser ex- 

 tent in a few other phyla. The rotifer body is of a 

 structural grade that includes several complete sys- 

 tems of organs, some of them more complex than 

 those of the flatworms, some less so. 



No large grouping of animals is more partial to 

 fresh waters than are the rotifers. Of some seventeen 



hundred described species, only about fifty are said 

 to occur solely or mostly in the sea, though common 

 fresh-water species are often carried into brackish 

 or salt waters and manage to survive there. Of the 

 marine forms nearly all live on shore bottoms. Only 

 two species have been seen afloat in mid-Atlantic. 

 The fresh-water rotifers also stay close to shore, 

 about 75 per cent of them living on the bottom or on 

 plants at the edges of lakes and ponds. Not more 

 than about a hundred species are freely floating 

 types, completely independent of any firm substrate. 

 A few rotifers live on the external surfaces of other 

 animals, as on the gills of crustaceans. Among the 

 parasitic species are Proales parasita and Asco- 

 morpha volvocicola, which enter colonies of the co- 

 lonial protozoan Volvox, living and breeding within 

 the spherical colonies and feeding on the members. 

 Drilophaga parasitizes fresh-water annelids, and 

 there are rotifers parasitic on protozoans, hydroids, 

 pond snails, and plants. 



Two rotifers ( at top ) , 



a gastrotrich and a kinorhynch 



Though many bdelloid rotifers are fully aquatic, 

 this group is the one most characteristic of lichens 

 and mosses. Their almost incredible capacity to sur- 

 vive when seemingly as dry as dust particles enables 

 them to live even in such intermittently wet places 

 as rain gutters and cemetery urns, moss-covered walls 

 or roofs, glaciers, rocks, and the bark of trees. Dry- 

 ing must occur gradually, as it does in the crevices 

 of moss, and the rotifer withdraws into the central 

 trunk region, puckering the two ends shut. The body 

 shrinks by loss of water and becomes more and more 



[139 



