86 OKHAMANDAL MARINE ZOOLOGY REPORT 



surface of the mantle and the palps is also sensitive, but not to greater degree than is 

 usual among other Lamellibranchs. 



Each of the pallial sense-organs (figs. 10, 11 and 13, S.o.) is an elongated tumid 

 body situated on the inner surface of each mantle close to the distal end of the pyloric 

 caecum. Innervation is by a strong branch of the posterior common pallial nerve, 

 which splits up into two principal branches immediately upon entering. The surface of 

 the swelling is covered with epithelium containing specialised sense-cells. The size of 

 these bodies is very much greater than in Margaritifera vulgaris, implying a function 

 of considerable importance. 



THE REPRODUCTIVE ORGANS. 



Placuna placenta is dioecious and the sex of each individual remains the same 

 from season to season throughout life ; no form of hcrmaphroditism occurs. 



In each sex the gonad or reproductive organ appears to the naked eye as an 

 irregular mass of yellowish tissue largely enveloping the stomach, liver, and intestine, 

 and pushing out irregular tumid lobes into various parts of the right mantle. No 

 sign is given that the gonad is paired. Even in very young specimens there is 

 absolutely no evidence in favour of such view, whereas the presence of a single genital 

 opening and the great asymmetry of the oi'gan point to the suppression of the left 

 member of the original pair. The one remaining is purely and simply an irregular 

 ramified mass closely associated with the course of the alimentary canal. It is 

 distinguishable as a central mass and a definite number of large lobes ; the first, as 

 before stated, envelopes and hides the greater portion of the stomach, liver and 

 visceral coil of the intestine, constituting the greater part of the outer tissue of the 

 main visceral mass. The lobulated masses which burrow in various directions into the 

 right mantle consist of two thin extensions into the dorsal or hinge lobes of the 

 mantle, one anterior and the other posterior to the central lobe bounded by the 

 cardinal teeth ; one long, narrow, greatly curved lobe following the entire course of the 

 pyloric caecum (fig. 13), first downwards along the anterior face of the adductor and 

 then turning posteriorly along its ventral aspect to a blind termination not far from 

 the posterior end of the gills ; another tumid and somewhat irregular mass envelopes 

 the rectum, with it forming the rectal lobe of the visceral mass ; the last of the large 

 sections of the gonad lies immediately posterior to the visceral mass (Go.p., 

 figs. 4 and 13). 



The last two lobes in conjunction with the posterior face of the visceral mass 

 bound the triangular cardiac chamber, except on the lateral aspects ; at the dorsal and 

 posterior angles these three masses are united and placed in communication with 

 each other by short and very narrow bridges of reproductive tissue ; the intercom- 



