336 MANUAL OF NATUKAL HISTOKT. 



lower surface, and often with the margin of their 

 disk simply fringed. The larger kinds, as Medusa 

 and Cydippe, are often seen languidly floating in 

 calm weather in our harbours, making their way 

 slowly by the regular expansion and contraction of 

 their umbrella-like bodies. In the Ciliograde kinds, 

 as may be observed in the globular Beroe, the organs 

 of progression are in the form of long filaments, 

 which enable their possessors to roll along through 

 the water in a very rapid manner, often appearing, 

 as they turn about, perfectly iridescent. One of them, 

 of singular transverse flattened form, the " Girdle of 

 Venus'" (Cesium Veneris), appears like a luminous 

 snake, as its long riband-shaped body, vividly phos- 

 phorescent, and often five or six feet in length, passes 

 along the surface of the water during a tranquil night. 

 Among the Physograde families, we find the well- 

 known Portuguese-man-of-war (Physalia pelagica), 

 those purple-crested vesicles seen by the voyager 

 floating so buoyantly in fleets upon the bosom of the 

 tropical seas. In the Cirrhigrade division we meet 

 with another delicate sailor in the form of the Por- 

 pita, with its purple tentacles and circular disk-like 

 skeleton ; here likewise we find Velella, also with a 

 cartilaginous support and of a lovely blue colour, but 

 with an oblique vertical plate on the horizontal sur- 

 face, which acts like a little lateen sail. In the 

 Diphydous Order are placed those curious double ge- 

 latinous animals the Salpce, which resemble two little 

 glassy bells, one fixed to the inside of the other ; 

 these perfectly transparent creatures are also pelagic, 



