SECT. in. 



VELELLA SPIRANS. 



titions, between which there is a series of concentric 

 chambers or galleries filled with air. The chambers 

 communicate with one another by apertures in the 

 dividing membrane ; they also communicate with the ex- 

 terior by perforations through the surface of the body. 

 Very long pneumatic filaments, that is tubes filled with 

 air, descend from the inferior surface of the float, and 

 pass through the lower plate of the disk into the water. 

 The disk is transparent, and appears to be white from 

 the air within it ; and it is marked by concentric rings 



Pig. 122. Velella spirans : 1, npper side ; 2, under-side. 



corresponding to the divisions in the air-vessel below. 

 The fringe-like limb that surrounds it is flat, flexible, 

 semi-transparent, and of the richest dark blue passing 

 into green, with a light blue ring ; it is very contractile, 

 and moves in slow undulations. The sail or crest is 

 thin, firm, and transparent, covered by a bluish mem- 

 brane ; its limb is dark blue, crossed by waving yellow 

 lines. 



An irregular microscopic network of vascular canals, 

 containing yellow matter, is seen in the soft substance 

 which covers the sail; it ends in a canal round its 



