92 



by a very short vessel whicli opens into the efferent 

 filamentar vessel. 



Respiration is effected by rythmical swallowing 

 movements of the mouth and co-ordinated liftings of the 

 opercula. The water swallowed passes from the pharynx 

 through the gill slits and over the surface of the gill fila- 

 ments. Probably movements of the latter in the mode 

 already suggested assist in this circulation of the sea 

 water. The gaseous interchange between the blood in the 

 respiratory lamellae and the water takes place through the 

 extremely thin walls of the latter and the walls of the 

 capillaries. Only two thin epithelia separate the two 

 liquids, and through these carbon dioxide passes from the 

 blood to the sea water, and oxygen from the sea water to 

 the blood. 



The respiratory area of the gills can be approximately 

 calculated. The lengths of the gill filaments vary, and 

 the greatest number of lamellae counted on any one side 

 of a filament was 225 ; probably 150 will represent the 

 average number to a side of a filament ; there are two 

 series of lamellae, of course, on each filament. Therefore 

 we have : — 



