THE COAST OF CALIFORNIA. 17 



at an angle to the longer diameter of the ellipsoid um- 

 brella or mantle, there is an oval, flat body called the float, 

 which is composed of two thin plates of horny character 

 united by a number of concentric partitions, the edges of 

 which are seen in Plate i, fig. 3. The concentric chambers 

 separated by these partitions are filled with air or gas, and 

 form an organ of flotation. They communicate with each 

 other by openings and exteriorly by a row of orifices 

 placed diagonally across the upper side of the float. The 

 float is placed left-handed across the umbrella, or if the longer 

 axis extends vertically, the upper end of the float is to the 

 left of the observer, the lower to the right. This was in- 

 variably the position of the float of all the specimens ex- 

 amined. 



On the upper side of the float there rises a thin chiti- 

 iious plate of triangular shape, the apex of which is above, 

 the longer side phiced slightly diagonally to the longer 

 axis of the float. Over the float is spread a thin mem- 

 branous body, a continuation of the mantle through which 

 ramifies a system of vessels. 



The two sides or edges of the triangular sail which are 

 free, are skirted by a continuation of this membrane form- 

 ing a contractile extension. The sail is carried upright as the 

 animal floats on the surface of the water. ^ 



The float of the specimens studied has a vertical crest 

 which like the float itself is "left handed ;" that is, when 

 placed before the observer with the central polyp turned 

 from him, the longer axis of the ellipse being placed ver- 

 tical, it has the upper portion of the crest on the left hand, 

 the lower on the right. The whole float has its axis left 

 handed as regards the umbrella. 



The color of the umbrella is yellow and blue, girt by 



> A reversal of the float so that the sail is below is generally fatal to the Velella. 



