8 MORPHOLOGY OP 



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 is 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 



