88 WILLIAM BATESON. 
As the body grows, the number of gill-slits increases. They 
are always added in pairs behind the last formed. The newest 
has a circular orifice, while the growth of the “ valves ” (fig. 4) 
from the dorsal margins of the anterior ones continues to 
modify their shape. From being circular they then become, 
first, kidney-shaped, then horseshoe-shaped, and next, by a 
diminution in width from before backwards; together with a 
great elongation dorso-ventrally, their openings are made 
U-shaped. When this condition is attained, the ‘ valve” 
continues to grow downwards, its free end lying inside the 
pharyngeal cavity, as will be described when the histology of 
the gills 1s treated of. Frequently, in contracted specimens, 
these valves are washed outwards through the gill-slit, and 
hang freely out in the water. This condition often occurs 
during life. The gill apertures are from the first strongly 
ciliated. The cilia move in a constant direction, driving a 
current dorsalwards on the anterior line of the U, then down 
the anterior margin of the “valve” and up the posterior, and 
finally ventralwards on the hinder edge of the gill-slit. The 
currents have the same course before the formation of the 
‘‘ valve,” viz. on looking at a circular gill-slit of the left side, 
if the animal’s head is directed to the observer’s left, the course 
will be round the aperture in the direction of the hands of a 
watch. By this current the water which passes in at the 
mouth is carried out of the pharynx; probably, therefore, the 
motion of the cilia is in a sort of spiral converging outwardly, 
and not circular as it appears to be on looking down upon a 
gill-slit. 
From the fact that the number of gill-slits varies with the 
length of the animal, together with the constant presence in 
the posterior branchial region of a regularly arranged series of 
gills in all stages from a complete U-shaped opening to a 
terminal one which is always circular, I am led to believe that 
these structures increase in number throughout the greater 
part, if not the whole, of the life of the animal. The greatest 
number of slits which I have observed was fifty-seven pairs. 
Figures illustrating the development of the branchial 
