268 FUNCTIONS AND INSTINCTS. 



in fine weather, where they steer their little 

 barks through, between, and over its fluctuating 

 waves, and spread their membranous sails to the 

 soft breathing of the zephyrs. 



One of the most noted animals of the tribe is 

 known by the appellation of the Boreal Clio, 

 which, like the jelly-fish, has a gelatinous body, 

 is defended by no shell, and affords food to the 

 whales and other fishes, as well as to the sea- 

 birds. This animal is abundant in places that 

 suit it, and appears only during the warmest 

 hours of the day on the surface. 



Other genera of this Order are covered by 

 a shell or shells. Of this kind is the genus 

 Hyalcea, so named from its semi-transparent 

 shell, which wears the appearance of a bivalve 

 with soldered valves, the upper one being the 

 largest ; this difference of size of the seeming 

 valves causes an aperture through which the 

 animal sends forth two large yellow and violet 

 wings, or sails, rounded and divided at their 

 summit into three lobes. The head in this genus 

 is almost evanescent, so that both shell and head 

 exhibit an easy transition from the acephalous or 

 bivalve Molluscans to those which have a head. 

 When its wings or sails are unfolded it moves 

 with great velocity on the surface of the sea. 

 The animals of this Order, both from the beau- 

 tiful colouring of their filmy sails or wings, and 

 from their number and symmetry, are better 



