92 
by a very short vessel which opens into the efferent 
filamentar vessel. 
Respiration is effected by rythmical swallowing 
movements of the mouth and co-ordinated lftings of the 
opercula. ‘he 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 lamelle 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 lamelle 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 :— 
| 
_Holobranchs. | No. of Filaments.| No. of Lamelle. 
is | 88 x 4 99600 
ile | 78x 4 93600 
II. | ae 86400 
Feee werraree so 69600 
| ae 
| “Total ...| 349200 
} es ees ee Ja u 
1164 
