458 J. H. ORTON. 



Fig. 10). On entering the mantle cavity the path of the ingoing 

 stream is suddenly widened (see Fig. 14) and, as is the case in Crepi- 

 dula, the heavier particles drop out of the current. These particles, 

 however, are collected by the mantle cilia into a definite ciliated path, 

 the cause of the "untere Eiickenstrom" of Stenta, which conducts 

 them posteriorly to a point in about the middle of the inhalent 

 chamber (see Fig. 10), whence the intruding material is expelled at 

 intervals by sudden flappings of the shell-valves. This stream is 

 protected, or rather rendered possible, by the infolded mantle edges, 

 which shield it from the main stream. Those particles which fall on 

 the mantle in the posterior part of the inhalent chamber are washed 

 ventrally, and are either shot out of the mantle cavity at any point, as 

 indicated by the arrows in Fig. 10, or are caught up by the gill and 

 carried forwards towards the mouth. 



In the American oyster there is a similar, posteriorly-directed 

 ciliated path on the anterior half of the mantle, but according to 

 Kellogg (7) there is also in the posterior half of the inhalent chamber 

 a forwardly-directed current, which carries intruding bodies forward to 

 the point where the current in the anterior part deposits whatever 

 material it may have collected. The whole of the foreign particles 

 collected by the mantle are then expelled at the point where the two 

 paths meet, that is, in about the middle of the edge of the inhalent 

 chamber. 



In Mytilus and Cardium (see Stenta, 6) the ciliated path collects 

 particles from the whole of the mantle and washes them posteriorly 

 into the exhalent chamber, but here, as also in Glycimeris ghjcimeris, 

 the inhalent and exhalent apertures are more definite than in the 

 oyster, both apertures, however, being posterior (see Figs. 11 and 12). 

 The ciliated paths in Cardium and Mytilus are excellently arranged 

 for expelling intruding bodies, for in the natural feeding position these 

 animals lie with the ventral surface apposed to the substratum, and 

 the current enters the mantle postero-ventrally. Hence the whole 

 length of the mantle cavity is utilized for the weeding out of the 

 heavier particles, which on falling out of the current drop straight 

 into the ciliated paths. Moreover, there is in Mytilus in the dorsal 

 angle of the inhalent aperture a fold of epidermis forming a sort of 

 curtain (see Fig. 11, B) which prevents the ingoing current from im- 

 pinging directly on to the gills by directing it ventrally. In this way 

 there doubtless results a more effective selection of the coarser particles. 

 In Cardium a semicircular fold of the mantle between the inhalent 

 aperture and the posterior ends of the gills (see Fig. 12, B) doubtless 

 assists in the automatic selection of the heavier food-particles in the 



