ii4 VELELLID&. PART in. 



tentacles, and which is speedily dissolved by the power- 

 ful solvent juices in its digesting chamber. 



Among the numerous appendages attached to the 

 under-surface of the Physalia, there are bluish-green 

 velvety masses fixed to extremely small branching pro- 

 cesses from the body of the animal, which seem with a 

 microscope to consist of tentacles, polypites in various 

 stages of development, male reproductive capsules which 

 are never detached, and female buds that are developed 

 into medusiform zooids, and are presumed to become 

 free as in other cases. The Physaliidae are social animals, 

 assembling in numerous shoals in the warm latitudes of 

 the Atlantic and Pacific. They naturally have their 

 crest vertical, kept steady by their tentacles, which drag 

 down in the water ; but Professor Huxley has seen them 

 at play, in a dead calm, tumbling over and over. The 

 Physalia does not possess the power of emptying and re- 

 filling its float with air : it is doubted whether any of 

 the physograde animals have that power, but the subject 

 is still in abeyance. 



The Velellidse are little sailing members of the physo- 

 grade group. The Yelella spirans (fig. 122), a Mediter- 

 ranean species, has a body or deck consisting of a hollow 

 horizontal disk, of a firm but flexible cartilaginous sub- 

 stance, surrounded by a delicate membranous fringe or 

 limb half the width of the body. A triangular vertical 

 crest, formed of a firm transparent plate, also encom- 

 passed by a delicate limb, is fixed diagonally from one 

 angle of the disk to the other, but not on the fringe ; 

 and as the natural position of the Velella is to float 

 horizontally on the surface of the water, the crest is 

 exposed to the wind and acts as a sail. 



The float or air-vessel is flat, horizontal, and nearly 

 fills the whole body of the animal : it consists of two thin, 

 firm, and rather concave plates joined at their free edges, 

 and united also by a number of concentric vertical par- 



