Chap. 27 AN AQUATIC MISCELLANY 543 



sonal changes in rotifers, differences among species, and variations in the form 

 and activities of individuals within the same species. In the perennial ones, 

 parthenogenetic reproduction continues throughout the year although sexual 

 reproduction may also occur in spring and fall. In the summer species, par- 

 thenogenetic reproduction occurs in summer, sexual reproduction in the fall 

 and the species is carried over the winter in resting eggs (Fig. 27.9). In the 

 winter species, there is a large parthenogenetic population in winter and the 

 males appear in the spring. 



Cycles Changed Experimentally. The reproductive cycles are readily 

 changed experimentally by food and temperature. When carefully cultured 

 populations of rotifers (Brachionus pola) were kept on scanty food, partheno- 

 genetic females were produced. When the food was adequate and plentiful, 

 sexual females soon became superabundant. In other experiments, D. D. 

 Whitney fed rotifers (Hydatina) on colorless flagellate protozoans (Polytoma) 

 and obtained 289 successive parthenogenetic generations. By feeding them 

 only chlorophyll-bearing flagellates, he could obtain sexual females at any 

 time. 



Economies. The reduction of male individuals enables rotifers to produce 

 large populations with a minimum consumption of food. The only function of 

 male rotifers is the fertilization of the resting eggs and their brief lives, with 

 little need of food, are entirely adequate for this function. Parthenogenetic 

 females eat far more than males, but every one of them produces more. Rabbit 

 populations are scanty compared with those of parthenogenetic rotifers. 



Phylum Gastrotricha 



Some gastrotrichs are marine, but most of them live in fresh water and are 

 often among the minute organisms swept up from the pond shallows with a 

 fine collecting net. Beneath the microscope they can be seen swimming, creep- 

 ing, even leaping rapidly about among the protozoans and rotifers with which 

 they consort, and in some ways resemble. Unlike the rotifers, they have no 

 crowns of cilia, but on their ventral sides they have bands of them which 

 accounts for their gliding and explains the name Gastrotricha (Gr. gaster, 

 belly -f trichos, hair). On the dorsal side the cuticle is scaly or hairy (Fig. 

 27.10). The majority of fresh-water gastrotrichs have a pair of tubes at the 

 end of the body, the outlets for the cement which forms their temporary hold- 

 fasts. In fresh-water gastrotrichs, all reproduction is parthenogenetic; no males 

 have ever been discovered. 



Phylum Bryozoa 



Their Place in Nature. Bryozoans or moss animals are minute animals, 

 nearly all of them living in colonies that look so much like moss that the name 



