chap, vii.i MOLLUSC A. 255 



of the ducts of the renal organ, but in others the latter 

 are used as a means of passage for the genital 

 products, just as are the brown tubes in the Ge- 

 phyrea. In Spondylus the products are' discharged 

 into the renal cavity ; in Mytilus (the sea mussel) 

 there is a distinct genital duct, which opens, however, 

 on the same papilla as the renal ; while in Anodon 

 and others the two ducts are completely separated. 

 This use of. the renal ducts by the generative glands 

 is regarded by Hubrecht as a more primitive arrange- 

 ment, but it was, he thinks, preceded by one in 

 which the genital products first escaped into the peri- 

 cardium, whence they were taken up by the renal 

 organ. 



In the lower Cephalophora the renal glands are 

 paired, and either open separately, as in Dentalium, or, 

 as in Proneomenia, the ducts unite posteriorly ; in the 

 more differentiated Gastropoda we find that the 

 organ of one or other side is affected by that torsion 

 of the body, which has so pronounced an influence on the 

 development of all the other organs of these molluscs. 

 In the Pulmonata the external orifice is obscured 

 by opening into the cavity of the air chamber, but as 

 this is merely formed by the folding over and attach- 

 ment of one edge of the mantle, there is no reason to 

 suppose that there is any real change in its essential 

 morphological characters. Sometimes the terminal 

 portion of the gland has muscular tissue developed in 

 its walls, and in some Heteropods and Pteropods the 

 whole organ is capable of contracting. 



In the Cephalopoda there are either one 

 (dibranchiata) or two (tetrabranchiata) pairs of renal 

 organs (Fig. 105 ; rr). They are richly supplied with 

 blood-vessels, which enter into the lamelliform pro- 

 cesses that project into their interior ; they open by 

 a somewhat circuitous course into that portion of 

 the body cavity which surrounds the heart, and 



