VII MOLLUSCA—ORAL LOBES OF THE LAMELLIBRANGHIA 105 



its concave surface, the cilia being continued in a band all along the tentacle to its 

 base. Tentacles of this sort are found in all stages of development ; they rise 

 chiefly from the inner surface of the shield, and easily become detached or broken 

 off, and are then regenerated. They are no doubt chiefly useful as organs of touch, 

 and serve for seizing particles of food (Foraminifera, etc.). They may further assist 

 respiration in the absence of localised gills, by causing increase of surface. The 

 tentacles are innervated from the cerebral ganglion through the stalk of the shield 

 on which they stand. 



C. Cephalopoda. 



In Nautilus, there are on each side one tentacle above and one 

 below the eye. It is not improbable that these two tentacles cor- 

 respond with the two pairs of tentacles in the Gastropoda. 



IX. The Oral Lobes of the Lamellibranehia. 



The oral aperture of the Lamellibranehia is produced right and 

 left in the form of a groove, which runs backward along the surface 

 of the body to the anterior end of the base of the gill, or to some 

 point near it. This groove is bordered by two projecting ridges 

 above and below it. The two upper ridges, at the point where they 

 meet, form a sort of upper lip over the mouth, the lower ridges, in 

 the same way, forming a lower lip. The groove between the ridges 

 serves for conducting to the mouth the particles of food which are 

 swept past the gills by the cilia. 



The length of the groove is naturally determined by the distance 

 between the anterior ends of the gills and the mouth. 



The two ridges just described are continued posteriorly in the 

 shape of thin lamellae, which hang down into the mantle cavity. These 

 lamellae, between which the groove becomes a deep, narrow cleft, are 

 the oral lobes or labial palps of the Lamellibranehia. They are more 

 or less triangular, one side of the triangle forming the base by which 

 the lobe is attached to the body. 



In cases in which the gills lie far behind the oral aperture, the bases of these 

 lobes are long, but in others, where they begin near the mouth, the bases are short, 

 and each lobe then usually forms a long, free, pointed process. The surfaces of these 

 two oral lobes are ciliated, and, further, the surfaces which face each other, i.e. 

 which have the groove between them, are striated at right angles to their bases. 

 This striation is caused by parallel ridges, and gives the lobes a superficial resem- 

 blance to gills. The lobes contain blood lacunic, and it is probable that, besides their 

 chief function of conducting food to the mouth, they may assist in respiration. 



In certain forms, the free edge of the upper lip folds over that of the lower 

 {Ostrea, Tridacna) ; in others, the two edges are closely apposed and interlocked by 

 means of processes and folds {Pcdeit, Spondyhis), so that a closed cavity rises in 

 front of the mouth, into which the gi'oove brings particles of food from each side. 

 The edges of the upper and lower labial palps may even grow together (Lima). 



Nucula (Fig. 21, p. 14), in which the ctenidium lies far back, and has a very 

 small respiratory surface, may serve as an example of very highly developed oral lobes, 



