1 1 8 THE PORPITA. PART in. 



the rest become of a darker blue towards the limb ; the 

 tentacles are pellucid and bluish, and the three last rows 

 have little dark blue globes attached to them by slender 

 filaments. 



The Porpita has a horizontal air-vessel divided ver- 

 tically into air-chambers like the Velella, but they are 

 much more numerous. In a middle-sized Porpita, four or 

 five lines in diameter, there are twenty-three or twenty- 

 four air-chambers surrounding a central one, and eighty 

 or ninety pneumatic filaments, so that the animal is ex- 

 tremely buoyant. Brown matter, supposed to be a liver, 

 lies directly below the undermost wall of the air-vessel, 

 through which, as well as through the base of the ani- 

 mal, all the pneumatic filaments penetrate ; the greater 

 number go straight down into the water, but a portion 

 of them terminate in the walls of the polypites. 



A complete system of canals, ciliated internally, tra- 

 verses all parts of the animal ; and it may be presumed 

 that the cilia maintain its juices in a state of circulation 

 similar to that in the Velella ; and the functions of the 

 polypites, great and small, that are in connection with 

 the liver, are also similar to those of the Velella. The 

 Porpita is armed with thread-cells like all the class. 

 The central polypite is sterile and nutritive ; the small 

 ones are both nutritive and reproductive : buds spring 

 from their stems, which become independent male and 

 female medusiform zooids, swim away from their parent 

 and produce abundance of eggs, whence a new genera- 

 tion of Porpitse arise. 



In this singular class of fresh-water and oceanic Hy- 

 drozoa, the internal cilia, aided by the contraction of the 

 walls of the body, are the sole means provided by nature 

 for the circulation of the fluids. 



