120 



found that when the ova were quite ripe the spermatozoa 

 had either been shed or were not quite ripe, some being 

 found with ripe ova and sperms not ripe and others just 

 the converse. The same applies to P. mainmus, and the 

 difference in the time of maturity of ova and sperms 

 cannot be more than a few days. 



Specimens of P. oi^ercularls with ripe gonads have 

 been obtained at all times of the year on the scallop beds 

 in the Firth of Forth, the maximum of reproductive 

 activity being, however, in July and August. Pecten 

 maxiimis has been dredged off Port Erin with ripe gonads 

 in December, April, May and throughout the summer, 

 and in the same condition off Belfast Lough in February 

 and March, so that the reproductive activity extends at 

 least over the greater part of the year. 



The gonads have one opening on each side into the 

 renal organ, situated near the pericardial end, at the 

 level of the prominence on each side of the visceral mass 

 near the attachment of the lower ends of the inner labial 

 palps. The ducts can be traced in serial sections, and 

 although the opening cannot be made out in dissections, 

 it is possible, by pressing the ripe gonad, to force the 

 products out into the renal organ, though care must be 

 taken not to force them through one of the veins leading 

 from the visceral mass, which may be easily mistaken for 

 the oviducts. The products of the gonads must be poured 

 directly into the sea through the renal organ, and thus 

 fertilisation takes place externally. 



The gonad consists of many branched tubuli (fig. 53, 

 Go. d.), bearing numerous almost spherical sacs, the 

 alveoli (fig. 53, Go. al. s. and Go. al. o.). The sexual 

 products, spermatozoa or ova according to the position, 

 arise by the proliferation of the cells forming the germinal 

 epithelium on the walls of the alveoli. As the gonads 



