MOLLUSCA. 261 



tained a considerable size. In its hinder part a provisional 

 paired byssus-gland is developed from the epidermis in Cyclas 

 and other forms. In other cases, e.g. Mytilus, the byssus-gland 

 is permanent. The byssus-gland occupies very much the position 

 of the Gasteropod operculum, and would appear very probably 

 to correspond with this organ. The anterior part of the foot is 

 usually ciliated. 



The gills appear rather late in larval development along the 

 base of the foot on either side, between the mantle and the foot 

 (fig. 1 20, br). They arise as a linear row of separate ciliated 

 somewhat knobbed papillae. A second row appears later. The 

 two rows give rise respectively to the two gill lamellae of each 

 side. 



The further history of the development of the gills has been studied by 

 Lacaze Duthiers (No. 297) in Mytilus. The first row of gill papillae formed 

 becomes the innermost of the two lamellae of the adult. The number of 

 papillae goes on increasing from before backwards. When about eleven 

 have been formed, their somewhat swollen free extremities unite together, 

 the basal portions being separated by slits. 



The free limb is formed by the free end of the gill lamella bending upon 

 itself towards the inner side and growing towards the line of attachment of 

 the lamella. The free limb is at first not composed of separate bars, but of 

 a continuous membrane. Before this membrane has grown very wide, 

 perforations are formed in it corresponding to the spaces between the bars of 

 the attached limb. 



The outer gill lamella develops in precisely the same way as, but some- 

 what later than, the inner. The rudiments of it appear when about twenty 

 papillae of the inner lamella are formed. Its first papillae are formed near 

 the hind border of the inner lamella, and new papillae are added both in 

 front and behind. Its free limb is on the outer side. 



In Mytilus the two limbs (free and attached) of each bar of the gill are 

 joined at wide intervals by extensile processes, the ' inter-lamellar junctions,' 

 and the successive bars are attached together by ciliated junctions. In 

 many other types the concrescences between the various parts of the gills 

 are carried much further ; the maximum of concrescence being perhaps 

 attained in Anodon and Unio 1 . 



Large paired auditory sacks seem always to be developed in 

 the foot; and clearly correspond with the auditory sacks in 

 Gasteropoda. 



1 R. H. Peck, "Gills of Lamellibranch Mollusca." Quart. J. of M. Science, 

 Vol. xvn. 1877. 



