AN AQUATIC MISCELLANY 



541 



Chap. 27 



not extend beyond the stomach in either sex; in others, it is incomplete only 

 in the males. Undigested remains are discharged from the mouth as they are 

 in hydra. 



Nitrogenous waste is removed by means of ciliated cells (flame cells) 

 located at intervals along the two excretory tubes. These primitive kidneys 

 that extend backward beside the digestive tube open into the contractile vesicle. 

 The vesicle discharges relatively large amounts of water into the cloaca. Thus, 



Fig. 27.8. Left, rotifers (Brachionus) holding on to Daphnia by their 

 sticky toes, and collecting particles of food from the water as they ride. Right, a 

 fierce carnivore (Dicranophorus) eating its way into one of its neighbor caldo- 

 cerans. (Courtesy, Myers, "What is a Rotifer?" Nat. Hist. 25:221, 1925.) 



the excretory system is a water balancer just as the contractile vacuole is in the 

 ameba and as the kidneys and urinary bladder are in the frog and higher 

 animals. 



The main part of the nervous system is the brain and from it nerves pass to 

 various organs. There are several sense organs, usually one or two red eye- 

 spots, evidently strong tactile senses in the corona, and a pair of sensory tufts 

 on the sides of the body. The sense of touch must be elaborate in Floscularia 

 {Melicerta ringens) which builds its exquisite case with great precision of uni- 

 formly rounded microscopic pellets (Fig. 27.5). 



Reproduction and Life Cycle. Female rotifers have a single ovary, a yolk 

 gland that supplies the eggs with food, and a short oviduct that carries them to 

 the cloaca in which they are fertilized (Fig. 27.6). Male rotifers are incom- 

 pletely developed except for the reproductive system. In some species, there 

 are no males. All eggs have the diploid or double number of chromosomes and 

 develop without fertilization into females. 



An annual succession of generations typical of many summer rotifers (hav- 

 ing parthenogenetic generations in summer) is outlined in Figure 27.9. The 

 chief peculiarities of rotifers are due to the presence of the diploid number of 

 chromosomes in the eggs of the parthenogenetic female-producing females; 

 and of the haploid or single number of chromosomes in the eggs of the sexual 

 females. Parthenogenesis and diploid and haploid numbers are explained in 

 Chapters 18 and 20. The reproductive cycle has a seasonal rhythm. A stem 

 mother produces a generation of females, parthenogenetically. These are suc- 

 ceeded by several generations of females, an enormous population, all likewise 



