SIPHONOPI10KA ; 1'iiYSALIA. 541 



would appear that they have considerable power over these or- 

 gans ; either forcing out the air contained in them, or compress- 

 ing it into a much smaller compass, when they wish to sink ; and 

 distending the sac by some unknown means when they desire to 

 rise. We seldom meet with anything like radial symmetry in 

 this group. The forms of the species are extremely variable and 

 irregular ; but there is usually a correspondence between the two 

 sides of each individual. A well-known example of this group 

 is the Physalia utriculus, commonly 



termed the Portuguese Man-of-war. 

 This possesses a single large air-sac, 

 which is surmounted by a sort of crest 

 (b), possessing a greater degree of firm- 

 ness than the rest of the structure, and 

 elevated entirely above the water, when 

 the animal is floating at the surface, so 

 that when exposed to the influence of 

 the gentle breeze, the animal is wafted 

 by its means from place to place. The 

 air-sac itself possesses considerable mus- FIG. 727. PHYSALIA. 

 cular power. It is provided with two orifices (, c), one at each 

 extremity, through loth of which air is forced out when the bag 

 is compressed by the hand ; each of these orifices is provided 

 with a little circular muscle, which usually keeps them closed, 

 but which allows of their dilatation during the continuance of 

 the o itward flow of air. It is not improbable that the alteration 

 in the specific gravity of this animal required for its sinking in 

 water, is effected partly by the explosion of air in this man- 

 ner, and partly by the compression of the remainder. Beneath 

 the air-sac there is a mass of short flask-shaped appendages, 

 which hang down from the under side of the air-sac, and are 

 terminated by suckers, with an orifice in each. These are re- 

 garded as the individual Polypes of the structure. Whilst the 

 lower surface of the air-sac is not itself above six inches from one 

 end to the other, the tentacu a sometimes hang down like fishing- 

 lines, to an extent of sixteen or even eighteen feet. They 

 gen: ra.ly possess an active stinging power, and are also very 

 TOL. IT. PP 



