LOCOMOTION AND FEEDING 



275 



are joined to each other ; the result of this is a large space between 

 the gills and the body, which, since the anus opens into it, is 

 called the cloacal space ; it opens to the exterior by the dorsal 

 siphon. The mantle, gills, and palps are all covered with cilia 

 of various lengths and direction of beat. These maintain a current 

 of water which comes in at the ventral or inhalent siphon ; 

 particles in the water are sorted by size, some being taken to 

 the mouth, and others rejected through the inhalent siphon. 

 Cilia in the dorsal or exhalant siphon drive out faeces and excreta. 

 The muscular closing of the valves ejects water rapidly from both 



o. fc. 



Fig. 202. — A, A horizontal section through a gill of the swan mussel, under low 

 magnification ; B, a single filament of the same, more highly magnified. 



af.c, Abfrontal cilia ; bl.sp., blood spaces ; f.c, frontal cilia; HI., filaments ; i., side of filament towards 

 interlamellar space ; i.f.j., interfilamentar junction ; i.l.j., interlamellar junction ; i.l.sp. interlamellar 

 space ; Lc, lateral cilia ; l.f.c, laterofrontal cilia ; o., outer side of filament ; sk.r., sections of the 

 chitinous skeletal rods which support each filament. 



siphons, and takes place both when the animal is disturbed and 

 when noxious water enters the mantle cavity. The major part 

 of the gas exchange takes place through the surface of the mantle. 



GENERAL ANATOMY AND ALIMENTARY SYSTEM 



The swan mussel is a coelomate animal, intermediate between 

 the earthworm and the crayfish in respect to its coelom and 

 haemocoele. It has a perivisceral coelom, situated in the back, 

 enclosing the heart and rectum and communicating with the 

 exterior by an excretory tube on each side. This space is the peri- 

 cardial cavity (Fig. 203). In the rest of the body the organs are 

 separated by blood sinuses, the circulation being an open one. 

 The cavity of the gonads represents a part of the coelom. ]\Iost 

 of the viscera lie in the upper part of the body, known as the 



