284 Records of the Indian Museum. [VOL. XXIT. 



in the shape of the shell though not in height. In Vivipara 

 oxytropis (Benson) Annandak found that while there was no 

 difference in the height of the shell of the two sexes in the adult, 

 the female shell was considerably broader than the male. In 

 Lecythoconcha lecythis (Benson) also large shells were of the same 

 height, but the whorls of the spire increased in size more gradually 

 in the female. In the genus Taia Annandale (1918, p. 137) found 

 that there was very little sexual variation in the shell in T. iiitha, 

 though he found specific differences in sexual variation in the 

 genus as a whole. In Lecythoconcha and Taia the young in utero 

 are relatively large as compared with Vivipara bengalensis but 

 this is not so in V. oxytropis. It appears probable, therefore, that 

 there is some reason other than the mere necessity of increased 

 space for the accommodation of young to account for this difference 

 in height of the shell in the two sexes, and a possible cause may be 

 found in the natural antagonism between bodily growth and the 

 attainment of sexual maturity. In Vivipara bengalensis and pro- 

 bably in manj' other species of mollusc in this country sexual 

 maturity is attained at a \txy early date, ^^■hitfield (1882) states 

 that examples of Limnaea {Buliinnaca) mcgasoma Say, which he 

 hatched out from the egg and managed to rear successfully in the 

 United States, America, became sexually mature at the age of one 

 year. According to Baker (igir, p. 50) " the duration of life in the 

 family Limnaeidae is from three to four years, full maturity being 

 reached in about two j^ears," and though I have no information 

 regarding other genera in temperate regions it is probable that 

 they much resemble the I,imnaeidae in this respect. In this country 

 and in Egypt, however, the general condition is vastly different. 

 Manson-Bahr and Fairley (1920, p. 65), who were engaged in inves- 

 tigations regarding vSchistosomiasis in Egypt, state that in that 

 country examples of Bullinus and Planorbis become sexually ma- 

 ture at the early age of three months, and the same undoubtedly 

 occurs in examples of Vivipara bengalensis in India. Dissection of 

 fifty examples belonging to the 1920 brood, during July, showed 

 thai ahead}' manj' of them were sexually mature, and this was 

 especiallj' so in the males. In many cases the gonad was well 

 developed and was full of ripe spermatozoa. In the females, 

 only comparatively few were sexually mature. The smallest 

 sexually mature male measured 13 mm. in height, while the 

 smallest female with eggs and young in utero was 16 mm. Two 

 others of the same height had a quantity of seminal fluid in the 

 egg-shell gland, so that copulation had taken place. Assuming 

 that the rate of growth is equal in both sexes up to the onset of 

 sexual maturity, it would appear that the males become mature 

 at an earUer date than the females, and the antagonism between 

 growth and the attainment of sexual maturity occurs when the 

 males are smaller, thus producing a disparity in size between the 

 two sexes. In spite of their increased bulk females of the 

 age of two years are remarkably less fertile than those of only 

 one year old. In only three examples out of a total of fourteen 



