THE LAMELLIBRANCHIA 241 



into the kidney of the same side, not near the pericardial orifice, 

 but nearer to the external aperture, e.g. in the Anomiidae and 

 Pectinidae, or close to the external orifice, as in Area.. In other 

 cases the gonad and the kidney open together into a common slit or 

 cloaca (Ostraea, Cyclas, Fig. 218, g.o, and certain Lucinidae). Finally, 

 in those cases in which there is a separate generative aperture, it 

 may either be situated on a papilla common to it and to the renal 

 orifice (Mytilus edulis), or, as is most frequently the case, it may be 

 in the immediate neighbourhood of the renal orifice, and like it, 

 situated to the outside of the visceral commissure (Fig. 242, (6) ?/). 

 When normal hermaphroditism occurs in the Lamellibranchia, 

 it may exist in one of the following different forms. In the first 

 each gonad is entirely hermaphrodite throughout its extent ; that 



Cyclas, left-side view, after removal of the left pallial lobe, gill, and auricle. a.o, anterior 

 adductor ; a.o, auricula-ventricular orifice ; a.r, anterior foot retractor ; a.s/anal siphon ; br.s, 

 branchial siphon ; /, foot ; g, tight gill ; g.o, genital orifice ; in, intestine ; k, left kidney ; I, 

 liver ; ov, ovary ; p.a, posterior adductor ; p.r, posterior foot retractor ; t, testis ; v, heart- 

 ventricle ; v.g, visceral ganglion and commissure (posterior part). 



is to say, uniformly composed of acini capable of producing ova and 

 spermatozoa simultaneously or successively. This condition is 

 found in Ostraea edulis, 0. angasi, 0. plicata, 0. lurida (other species 

 of Ostraea, viz. 0. virginica, 0. glomerata, and 0. angulata are 

 dioecious), Kellya, and Lasaea, In the second form there are male 

 and female acini lying side by side throughout the whole extent of 

 the gonad, e.g. Tridacna, Cardium oblongum, and C. norvegicum. In 

 the third form the gonads are differentiated into regions of different 

 sex, the anterior region being male and the posterior female (Fig. 

 235, t, ov\ but these are not separate from one another, and have a 

 common duct and a single orifice : this is the case in Peden herma- 

 phroditus, P. maximus, P. jacobaeus, P. opercularis, P. glaber, P. irra- 

 dians, and P. flexuosus (P. inflexus and P. varius are dioecious). The 

 same arrangement is found in the Cyrenidae (Cyclas, etc.), in which, 



16 



