302 MOLLUSCA [CH. 



be attached to the shell near the umbo pull the shell downward 

 and help to plough a furrow in the mud. The animal moves by 

 forcing out the foot and wedging it in the mud in front and then 

 drawing the body after it. 



At the sides of the body on each side we find the branchia or 

 gill, or ctenidium, which as in the Gastropoda consists of a hollow 

 axis bearing two rows of plates. The ctenidium is, however, highly 

 modified in Unio. The axis is attached high up to the side of 

 the body in front but projects freely into the mantle cavity behind. 

 The plates have become narrowed so as to form long filaments, 

 and the ends of each row are bent up and are, in the case of the 

 outer row, fused to the mantle lobe. The bent-up ends of the 

 inner row are joined to the foot in front and to the corresponding 

 parts of the ctenidium of the other side behind, but in the middle 

 they are free, at least in some species (Fig. 138). Successive fila- 

 ments of one row are welded together into a plate, called a lamella, 

 by the fusion of their adjacent edges, leaving only occasional holes 

 for the percolation of the water, so that individual filaments appear 

 like ridges on a ploughed field (Figs. 137, 139). The descending 

 and bent-up ends of the same filament are tied together by cords or 

 narrow plates of tissue traversing the space between them. These 

 cords and plates are called inter lamellar junctions, since they 

 unite two lamellae. The pieces of tissue uniting the filaments are 

 called interfilamentar junctions, or collectively, subfilamentar 

 tissue. Gill -plate is the name given to the whole mass composed 

 of one row of V-shaped filaments : there is thus an outer and an 

 inner gill-plate on each side, and each gill-plate has two lamellae 

 formed from the descending and ascending limbs of the filaments, re- 

 spectively (Fig. 133). It is this peculiar modification of the ctenidia 

 which has suggested the name Lamellibranchiatafor the class. 



Each V-shaped filament is clothed on its outside, that is, the side 

 looking away from the concavity of the V, with high ectoderm cells 

 carrying powerful cilia; these are of two kinds, and from their 

 respective positions are termed frontal and lateral cilia re- 

 spectively. By the latter a strong current of water is produced 

 entering between the posterior borders of the mantle lobes, which 

 normally gape slightly. On this current the animal depends both 

 for respiration and nutrition, since the food consists entirely of the 

 minute animals and plants swept in with the water. The normal 

 position of the Mussel is to have the anterior end deeply embedded 

 in the sand or mud and the posterior end protruding; the animal 



