The mantle lobes are developed as lateral outgrowths of the body: they usually have a considerable extension before they are covered by the shell. In Anodon and Unio the larval mantle lobes are, however, formed in a somewhat exceptional way, and are from the first completely covered by the valves of the larval shell. The larval mantle lobes and shell in Anodon and Unio are subsequently replaced by the permanent structures. [FIG. 118. AN EMBRYO OF PISIDIUM PUSILLUM. (From Lankester.) _f._ foot; _m._ mouth; _ph._ pharynx; _gs._ bilobed stomach; _pi._ intestine; _shs._ shell-gland.] The adductor muscles are formed soon after the appearance of the shell. The posterior sometimes appears first, _e.g._ Mytilus, and at other times the anterior, _e.g._ Cardium. The foot arises in the usual way as a prominence between the mouth and anus. In comparison with Gasteropoda it is late in appearing, and in many cases does not become prominent till the shell has attained 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. 120, _br_). They arise as a linear row of separate ciliated somewhat knobbed papillæ. A second row appears later. The two rows give rise respectively to the two gill lamellæ 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 papillæ formed becomes the innermost of the two lamellæ of the adult. The number of papillæ 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 somewhat later than, the inner. The rudiments of it appear when about twenty papillæ of the inner lamella are formed. Its first papillæ are formed near the hind border of the inner lamella, and new papillæ are added both in front and behind. Its free limb is on the outer side.