SEXl'AL ORGANS. 



45 



on the edge of the anus, and by i)ros,sure its brownish contents 

 may be made to issue. The function of this, as well as of the 

 jHirpIe gland is nuknown. 



Sexual Oryans. 



The sexes are distinct in the prosobrancliiates : the organs 

 are, however, very simi)le and so alike in structure, tliat frec^uently 

 the sex of the gland can only be determined by microscopical 

 examination. Usually a germ-secreting gland is imbedded in 

 the liver, from wliich an efferent duct opens at the right side into 

 the mantle cavity. In most cases the males may be readily dis- 

 tinguished by the large penis, which is placed at the right side 

 of the head behind the eyes. The shell in the female is generally 

 more inflated than in tlie male. 



Female Organs, (pi. 3. fig. 19.) The ovary discharges into 

 a much looped oviduct and the latter dilates into a gut-like 

 uterus — the last portion of which, on account of its muscular 

 wall may be regarded as a vagina. There is sometimes at the 

 commencement of the uterus, or at its connection with the 

 vagina, a seminal pouch, but other ai)pendicular organs are seldom 

 present. 



The last portion of the uterus or even the whole of it, and the 

 vagina lie in the respiratory cavity, to the left by the side of 

 the rectum and nearest to the abdominal wall. The sexual open- 

 ing is accordingly found to the left of the anus, but usually far 

 behind it. Sometimes the uterus is split throughout its entire 

 length and its folds formed l)y the longitudinal and transverse 

 plaits, consequentlj' lie freely exposed in the respiratory cavity. 

 Lacaze-Duthiers has so described it in Vermetus. 



There is scarcely anything to be said in regard to the eggs of 

 the prosobrancliiates, generally. Where their development can be 

 seen, a distinct germinal vesicle and germinative dot are present ; 

 but when they leave the ovarium the yolk granules are present 

 in such numbers as to conceal the before-mentioned structure. 

 In the oviduct, or quite above the uterus, the eggs come into 

 contact with the zoosperms, which arc occasionall}' retained at 

 this point in a spermatheca. Further down in the uterus fertili- 

 zation could no longer be effected as it here becomes enclosed in 

 a tough albumen, and finalh' is covered, usually many together 



