THE SWAN MUSSEL 



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are continuous with the labial palps, which are a pair of 

 triangular flaps, one outside the other, on each side of the 

 body, the outer palps being joined in front of the mouth 

 by the upper lip and the inner behind the mouth by the 

 lower lip. The palps are ciliated, and their surfaces which 

 are towards one another are crossed by fine furrows, 

 which lead upwards towards the groove between them. 

 This is continuous with the corners of the mouth, and the 

 whole forms an apparatus by which small organisms and 

 other fine organic particles are gathered from the water 



Fig. 224. — A, A horizontal section through a gill of the swan mussel, 

 under low magnification ; B, a single filament of the same, more 

 highly magnified. 



bl.sj>., Blood spaces ; y?/., filaments ; /., side of filament towards interlamellar space ; 

 i.f.j., interfilamentar junction; ?'././., interlamellar junction; i.l.sp., inter- 

 lamellar space; o., outer side of filament; sk.r., sections of the chitinous 

 skeletal rods which support each filament. 



in the mantle cavity and swallowed. Each of the gills 

 consists of two lamella continuous along their ventral 

 edges. As there are two gills on each side of the body, 

 there are on each side four lamellae. Each lamella is 

 composed of very numerous vertical filaments whose inner 

 sides (that is, the side of each towards the other lamella) 

 are fused together at irregular intervals, so as to form a 

 ribbed plate pierced by numerous openings between the 

 ribs leading into the interlamellar space of the gill. The 

 filaments of the two lamellae of a gill are continuous, each 

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