44 P.I,O0^[ER : THE OniGIX AND FT'XCTIOX OF THE FOt'RTII APERTURE. 



tliat in the animals possessing the fourth aperture the concresced ventral 

 portion is comparatively of considerable length. Pelseneer '^' calls atten- 

 tion to this feature. It must, hoAvever, be observed that there are genera 

 belonging to these families having proportionately the same extent of 

 ventral concrescence, in which the orifice does not occur. 



I have recently attempted to show that Solen vagina is a more primi- 

 tive form than Ensis ensis or E. siliqua, that Culfellus pellticidus lies 

 between S. vagina and E. siliqun. ; and, further, that the positi(-)n of 

 Ceraf isolen legumen is between ^S'. vagina and C. jeUiir.idus. S. vagina- 

 lias no fourth a])erture, neither has C. legnmen, but in the latter the dorsal 

 and ventral surfaces of the ])e(lal aperture carry a tentacular fringe, and 

 extend further posteriorly. In ('. jjellucidiis what represents the fourth 

 aperture is also present, but whether it is separated from the pedal aperture 

 I have not been able to determine, as this portion is so easily ruptured. 

 However, in the examination of a large number of specimens, I have not 

 found any concrescence of the mantle lobes l)etween these parts. In 

 E. ensis and E. siliqiia the fourth aperture is situated more posteriorly, 

 being about the centre of the ventral sui-face. There may l)e other genera 

 with the orifice between these two ])(>sitioiis, but up to the ])resent n(me 

 have come under my notice. From a, mori)hological jmint of view, it is 

 reasonal)le to infer that a portion of the ))edal apertiu'e first became special- 

 ised by develo])ing a tentacular fringe ; then this fringe extended pos- 

 teriorly, and the pallial walls coalesced, separating the fourth from the 

 pedal aperture, and finally the fourth aperture gradually proceeded further 

 posteriorly, until it attained a position favourable for the function it 

 originated for. 



If either ^S*. ensis or E. siliqiia is examined, it will be seen that the 

 fourth aperture is an elliptical opening bordered on both sides by a row of 

 tentacles, those on the one side alternating with those on the other, that 

 from it on the inside of each mantle lobe is a groove, and in these grooves 

 lie the distal portions of the labial palps, the ends of them being near to the 

 opening. The labial palps, and particularly the parts which form the b'ps, 

 are of considerable length. 



From experiments on E. siliqiia, I (tliserved that when particles of 

 carmine were placed near the aperture they were gradually drawn in, the 

 greater portion of them being passed between each pair of labial palps, anrl 

 accumulated near the bases, just in the track where the food travels from 

 the gills to the mouth. Now, as in all probability the labial palps assist 

 the gills in rejecting objectionable matter, as well as being the medium 

 by which the food is conveyed to the mouth, it appeared possible that the 

 greater portion of the carmine would be detected and ejected by the ex- 

 halent aperture, and that is what happened. Nevertheless, I found a certain 

 portion of the pigment had been passed anteriorly to tliis position, and 



1 Introduction 4 Tetude des mollusques. 



