SEA-FISHERIES LABORATORY. 193 



of very fine threads or filaments. These are hollow tubes 

 and blood from the heart flows down inside them. Each 

 filament is covered by an immense number of very small 

 hairs or cilia, and as long as the mussel is alive these are 

 waving backwards and forwards and causing a current 

 of water to be sucked into the shell. This current passes 

 over the gills towards the mouth. The food is strained 

 by the gills, caught up by the lips, and taken into the 

 mouth. At the same time the oxygen in the water passes 

 through the filaments of the gills into the blood and the 

 carbonic acid of the blood passes out into the water. 



..Muscle W _ Heart 



Shell 1 J>^*^ggs^*J^W _, , 



Byssus 



Diagram, of the structure of a mussel. Natural size. 



Breeding of the musse 1. — Mussels are 

 male and female. It is impossible to tell the sex except 

 by examining the animal microscopically. The eggs and 

 milt are contained in the mantle. Spawning takes place 

 in the spring, and immediately on spawning the fish 

 become thin and in poor condition because the eggs and 

 milt have been expelled from the body. 



The little animal which hatches out from the egg is 

 not at first like a mussel, and it swims about in the water. 



