216 LA MEL LIBRA NCHIA TA . 



tilus, the byssus-gland is permanent. The byssu.s-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 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 papillfe formed 

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

 papilla? goes on increasing from before backwards. When about eleven 

 nave 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 at- 

 tachment 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 

 papillie of the inner lamella are formed. Its first papillse are formed near 

 the hind border of the inner lamella, and new papillse 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 junc- 

 tions. 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 Anoclon and Unio 1 . 



Large pah~ed auditory sacks seem always to be developed in the 

 foot ; and clearly correspond with the auditory sacks in Gasteropoda. 

 Eyes are frequently present in the larva, though they disappear 

 in the adult. In Montacuta and other types a pair of these organs 

 is formed at the base of the velum on each side of the oesopha- 

 gus, not far from the auditory sacks. They are provided with a lens. 

 A row of similar organs is present in the larva of Teredo in front 

 of the foot. 



Cardium. As an example of a marine Lamellibranchiate I may take 

 Cardium pygmaetirn, the development of which has been studied by Loven 

 (No. 291). The ova, surrounded by a thickish capsule, are impregnated in 

 the cloaca. The segmentation takes place much as in Nassa (vide p. 83), and 

 the small segments gradually envelop the large hypoblast spheres ; so that 

 there would seem to be a gastrula by epibole. After the hypoblast has 

 become enveloped by the epiblast, one side of the embryo is somewhat flat- 



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

 Vol. xvn. 1877. 



