ORDER I. SIMPLICIA. 353 
ORDER I. ACALEPHA SIMPLICIA. 
This order comprises the genera Medusa, Cyanea, Rhizostoma, Astoma, 
Berve, and Cestwm, which are subdivided into several sub-genera. They 
swim by alternate contractions and expansions of the body. 
Mepusa. — This genus has a central disk on the upper surface, some- 
thing like the head of a mushroom, and sometimes called the umbrella. 
The margins of the umbrella, and those of the mouth in the middle of the 
under surface of the disk, are furnished with tentacula, very much varied in 
orm and size. 
These animals resemble a crystalline mushroom endowed with locomo- 
tive powers. The tentacles, however insignificant they may appear, are 
formidable weapons against all minute marine animals they come into con- 
tact with. Like those of the polypi, they are provided in many species 
with numberless small needles, which not only wound, but seem also to poi- 
son by the transmission of a corrosive liquid, which deprives the benumbed 
animal of all resistance. Several species, on being touched, produce a burn- 
ing sensation, so that they have been deservedly called Sea-nettles. 
They move by alternately expanding and contracting their umbrella-shaped 
disk, the convex upper surface of which is directed forward, while the fim- 
briated vessels and tentacles follow behind. 
This genus embraces all those Acalephw which have a true mouth on the 
under side of the disk; this mouth, however, is, in some species, a simple 
opening, and in others is placed on a peduncle. 
Cyanrea.— A central mouth and four lateral ovaries distinguish all the 
species of this group. 
C. Aurita, a widely distributed species, has at maturity four long arms, 
and another species, C. chrysaora, has the margin of the umbrella furnished 
with long tentacula, and rows of brown or yellow spots, forming rays on the 
convex surface. 
Ruizostoma. — Those Acalephie which have no central opening or mouth, 
and obtain their nourishment by suction through the tentacula, constitute 
this genus. These are common animals, often attaining a diameter of two 
feet, and a weight of twenty pounds. They are frequently left on sandy 
shores by the receding tide. They seem to be of a social nature, being met 
with in large congregations swimming in the same direction. 
Astoma. —The animals of this genus have no central mouth, no rami- 
fications of the peduncle, and no cavities for the ovaries. 
Beror. — These animals have a globular body, provided with salient ribs, 
extending from the centre of the upper surface to that of the under, and 
