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It will be remembered that in the common Mussel the 

 sexes are separate. Hermaphrodites, if they exist, must 

 be extremely uncommon as, in the examination of many 

 hundreds of specimens, none have been observed. There 

 is a slight preponderance of males over females, the ratio 

 in 218 specimens examined being 118 : 100 ; but it is 

 probable this is too low, as it is hard to avoid selection in 

 choosing the specimens for examination.* Except for a 

 slight difference in the colour of the mantle lobes there 

 are no external sexual characters. 



The gonad (PL I., fig. 2) is situated in the visceral 

 mass and mantle lobes ; the organ is paired and bilaterally 

 symmetrical, and consists of a branching tubular gland, 

 the efferent duct of which is situated on the summit of a 

 small papilla (PI. I., fig. 1, pp. gen.), about 1 mm. in 

 height, which lies posterior to the foot and immediately 

 beneath the gills {hr. B.), and can be easily seen by 

 reflectmg the latter and the mantle {Mn. B.). This duct 

 is richly ciliated for some distance internally, when the 

 ciliated wall is partially replaced by germinal epithelium. 

 Strips of columnar ciliated epithelium (PI. II., fig. 7, ep. 

 cil.) are present along the greater length of the tubules, 

 but the terminal portions are entirely lined by germinal 

 epithelium. Passing inwards these tubules branch 



* Since the above was written tlie tabulated results of the examination of 

 the specimens sent to the Laboratory by the Bailiffs have been referred to. 

 The portion of these records dealt with relates to the examination of a large 

 number of Mussels taken at random from most of the Mussel beds in the 

 district, and sent to the Laboratory in lots of a dozen each, and extends over 

 the years 1892-96. In order to eliminate as many sources of error as may be 

 possible, all those sheets in which the sex of one or more of the specimens is 

 regarded as doubtful, and also those dealing with Mussels taken about the 

 time of the "spent" period, have been rejected. This leaves a total of 821 

 specimens of which the sex was probably accurately determined. Of these 

 449 were male and 372 female, giving a ratio of males to females equal to 

 6 : 5 (very nearly). 



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