478 HYDROSTATIC ACALEPH^. PHYSALUS. 



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

 is the Physalus utriculus, commonly j,, 



termed the Portuguese Man-of-war. This 

 possesses a single large air-sac, beneath 

 which the digestive apparatus is disposed ; 

 and the sac is surmounted by a sort of 

 crest (>), which possesses a greater degree 

 of firmness than the rest of the structure, 

 and is elevated entirely above the water, 

 when the animal is floating at the sur- 

 face ; so that when exposed to the in- 

 fluence of the gentle breeze, the animal 

 is wafted by its means from place to place. FIG. ei 4. PHYSALUS. 

 The air-sac itself possesses considerable muscular power. It is 

 provided with two orifices (0, c), one at each extremity, through 

 both 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 outward 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 expulsion of air in this manner, and partly by the com- 

 pression of the remainder. No definite stomach can be found 

 beneath. 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. Whilst the 

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

 end to the other, the tentacula sometimes hang down like fishing 

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

 generally possess an active stinging power, and are also very 

 contractile, so that they are able to draw the prey which they 

 have attacked towards their point of origin. It would seem that 

 the short suckers are attached to the bodies of animals thus 

 entrapped ; and that the Physalus derives its nourishment by 

 imbibing their juices through the pores of these numerous 

 cirrhi. This animal is one of those most commonly observed by 

 voyagers, sailing in fleets upon the calm surface of the ocean, and 



