In the Coelentera, whatever subsequent changes of shape the little sac may undergo as it grows up to be polyp or jelly-fish, the original arch-enteron remains as the one cavity pervading all regions of the body.
In many cases, as, for example, in the Medusae or jelly-fish, the mesogloea may be so thick as to constitute the chief part of the body in bulk and weight.
In April 1874 a case was recorded of exposure in an open boat for 32 days of three men and two boys, with only ten days' provisions, exclusive of old boots and jelly-fish.
They include a jelly-fish, molluscs, prawns, crabs, &c., and were at first considered to form an isolated group found in no other of the African lakes; but this supposition has been proved to be erroneous.
Thus, about 1875, the distinction of Echinoderms from such radiate animals as jelly-fish and corals (see Coelentera), by their possession of a body-cavity ("coelom") distinct from the gut, was fully realized; while their severance from the worms (especially Gephyrea), with which some Echinoderms were long confused, had been necessitated by the recognition in all of a radial symmetry, impressed on the original bilateral symmetry of the larva through the growth of a special division of the coelom, known as the "hydrocoel," and giving rise to a set of water-bearing canals - the watervascular or ambulacral system.
