THE SWAN MUSSEL 317 



are separate in front, join behind to form a cloacal space which 

 communicates with the outside by the dorsal siphon. Into 

 this space opens the anus, which is placed above the posterior 

 adductor muscle. The surface of the gills is covered with 

 strong cilia, which set up a current of water through the 

 perforations of the lamellae into the interlamellar spaces 

 and so by way of the epibranchial and cloacal spaces to 

 the exterior at the dorsal siphon. The current is main- 

 tained by the entry of water at the ventral siphon, where it 

 passes over the tentacles, which test its purity, causing if 

 necessary a sudden closure of the valves which drives water 

 out at both siphons and washes any obnoxious substance 

 away. The continual renewal of the water brings fresh 

 supplies of oxygen to the gills and mantle, which are the 

 respiratory organs, and food which the gill-cilia pass on to 

 the labial palps. The outgoing current bears away carbon 

 dioxide, faeces from the anus, and excreta from the kidneys, 

 which, as we shall see, open into the inner epibranchial 

 chambers at their front ends. 



The swan mussel is a ccelomate animal, intermediate 

 between the earthworm and the crayfish in 

 Anatomy and respect to its ccelom and hsemocoele. It has 

 Alimentary a perivisceral ccelom, situated in the back, 

 " ■''"' ''"'' enclosing the heart and rectum and communi- 



cating with the exterior by an excretory tube on each side. 

 This space is known as the pericardium. In the rest of 

 the body the organs are separated by blood sinuses, the 

 circulation being an open one. The gonads represent a 

 part of the coelom. Most of the viscera lie in the upper 

 part of the body, known as the visceral hump, but the 

 gonads and intestine lie in the soft region of the foot. The 

 mouth leads into a gullet, which passes upwards into a 

 moderate-sized stomach behind the anterior adductor 

 muscle. Into the stomach is poured by several ducts the 

 secretion of a large digestive gland which surrounds it. An 

 intestine starts from the lower side of the stomach, takes 

 several coils in the soft upper part of the foot, turns 

 upwards, and runs straight backwards in the middle line of 

 the upper part of the body to the anus. The straight 

 part of the intestine is known as the rectum. It lies in 

 the pericardium surrounded by the ventricle of the heart. 



