PHYLUM TROCHELMINTHES 123 



anterior portion of the digestive tube serves as a suspensory ligament 

 for the testis. (Miller, 1931.) 



Cloacal fertilization is not apparently the rule, since in many 

 cases the male apparently bores into the body wall and the sperms 

 evidently pass through the wall of the oviduct in order to fertilize 

 the eggs. (Gamble, Camb. Nat. Hist.) Males hatch from the 

 small eggs of the mictic females. 



In the female the large yolk-gland or vitellarium^ which usually 

 consists of eight cells, is found on the ventral side of the stomach. 

 The true ovary consists of many small, rounded cells, the posterior 

 one enlarged and receiving a shell just before it is extruded. 



Rotifers are especially interesting to us on account of their use 

 in experiments on the alternation of parthenogenetic and bi-sexual 

 development. Whitney, Shull, Luntz and others have studied the 

 influence of food and other factors on the control of the reproductive 

 cycle in rotifers (Figure 52).' 



Recalling Loeb's discovery that in the Echinodermata the ex- 

 traction of water from the eggs by hypertonic solutions would start 

 developmental processes, Jacobs (1909) suggested that in certain 

 rotifers desiccation is able to bring on reproduction. Hickernell 

 (1917), studying Philodina roseola^ found that the dried rotifer 

 had an integument thinner than that of the undried specimen, and 

 that an increase in the number of ovarian nuclei occurred at the 

 very beginning of the drying process, while the animal was recover- 

 ing. But Whitney (1930), reporting on the hatchability of eggs of 

 Hydatina senta stored for twenty-two years, found that the fecundity 

 of individuals hatching from fertilized eggs after this long period of 

 desiccation was lowered, the mothers from old eggs producing an 

 average of thirty less daughters than the controls. 



It was shown by dal Bianco (Jour. Exp. Zool., vol. 39, no. i, 

 1924) that HCl and FeSOa brought about a notable acceleration of 

 the life cycle of Proales felisy even with concentrations that induced 

 a marked mortality. 



Jennings and Lynch (1928) studied the origin of individual 

 differences during parthenogenetic reproduction under constant 

 environmental conditions. Their studies seem to indicate that 

 differences in the length of life of rotifers result from the interaction 

 of rhythmic processes of digestion and reproduction. They found, 



1 Consult Shull, A. F., Determination of types of individuals in aphids, rotifers 

 and cladocera. Biol. Rev., 1929, vol. 4, no. 3, pp. 218-248. 



