Jelly Fish, the popular name of the aca-lephan class of radiated animals, or medusae including the orders hydroidae, discophorae, and ctenophorae. The body is transparent and jelly-like, disk-shaped, with the mouth downward and in the centre of the enclosed cavity, from which hang down appendages varying in number, length, and purpose. In the genuine medusae, of which the sun fish (aurelia), so common on our beaches after storms or floating in our waters in the summer, is a good example, the body is so largely made up of water that on drying it is reduced to a mere film of membrane; they would hardly be seen in the water were it not for their beautiful colors. The digestive cavity is more complex than in the polyps, being excavated in the substance of the body with branches ramifying in various directions; the stomach seems to perform the office of a heart, distributing the products of digestion over the system, and the food, arriving at the periphery, escapes by as many openings as there are traversing tubes; on the free margin are generally numerous minute tentacles, forming beautifully delicate appendages, which absorb water into the marginal canal in contact with the food; digestion is rapidly performed; the circulation of the digested materials is irregular, sometimes in one direction and sometimes in another.
The bunches of colored eggs generally hang outside the tentacles which surround the mouth; in some, red specks between the tentacles have been conjectured to be eyes. The common jelly fishes move by the alternate contractions and dilatations of the gelatinous disk; others, like the Portuguese man-of-war (physalia), have a large vesicle which supports the whole community at the surface of the ocean, motion being effected by the numerous contractile tentacles and the contractions of the air bladder; others (the ctenophorae or beroid medusas) move by means of vertical series of swimming appendages resembling the fins of a crab. This class presents the curious phenomena of alternate generation, illustrated by Steenstrup, Sars, and others, noticed also in other classes of the animal kingdom, especially the helminths or entozoa. The tubularia, common in pools left by the tide, a hydroid growing in tufts like small shrubs, hangs like a flower from a slender tube, with the mouth surrounded by tentacles, each animal connected with the rest of the community, and each mouth receiving nutriment for the whole; the young of this hydroid do not resemble the parent, but are little, delicate, translucent jelly fishes, like tiny cups from which hang down four long threads, and a proboscis at the end of which is the mouth: by the side of the buds branching out from the parent hang bunches of little spheres from which the jelly fishes are produced; along the proboscis of the floating cup are other spheres or eggs, from which are produced little pear-shaped bodies, which become attached and grow into the first mentioned branching hydroid.
It will thus be seen that the grandchild resembles the grandparent, and the hydroid is reproduced through a generation of jelly fishes into a hydroid again; if the first be a coryne, the jelly fish would be a sarsia. Some small single hydroids, not more than half an inch high, produce some of the largest jelly fishes; as the one which by subdivision into saucer-like contractions forms the ephyra, with a marginal fringe of tentacles. In our common white sun fish, the four cres-centic rosy figures, forming a cross by their union in the centre, are accumulations of eggs. Some of the jelly fishes in our waters formed from these self-dividing hydroids are as large as the largest wash tub, with tentacles extending 20 or 30 ft.; these are of a deep claret color, and possess in a remarkable degree the stinging or nettling property which has given the scientific name to the class. In the Portuguese man-of-war, some of the community move the whole establishment, some secure prey with their lasso cells and eat and digest for the family, and some produce the buds from which the young jelly fishes arise; and none of these take up or interfere with the work of the others. In the same way the hydroid cam-panularia produces the jelly fish tiaropsis, with its edge beautifully fringed.
Some very handsome jelly fishes do not originate from any hydroid, but reproduce themselves in the usual way by eggs. For the greater part of the year the eggs remain torpid, then a polyp-like vegetation arises, with buds which flower-like become rapidly developed into more highly organized free jelly fishes; these animal flowers, as they have been called, are so sensitive that they are instantly killed by a change from salt to fresh water. They are very voracious, feeding upon minute fishes, crustaceans, almost any small marine creatures, decaying animal or vegetable matters, and even their own species; they move with the rapidity and elegance of birds of prey, securing their victims with precision by means of their nettle-armed tentacles, and performing these acts in a manner which would hardly be expected in a transparent mass of jelly. A nervous system is present; and the form is in many capable of remarkable changes. For details on their structure, see Forbes's work on the British naked-eyed medusae; two papers by Prof. Agassiz in the "Memoirs of the American Academy of Arts and Sciences," vol. iv., part 2, 1850; and vol. iii. of Agassiz's " Contributions to the Natural History of the United States" (4to, Boston, 1860). The well known phosphorescence of the ocean is largely due to the light emitted by jelly fishes, shining like globes of fire, sparkling like stars, or diffusing a pale luminousness; this is most remarkable when the water is agitated by a vessel's keel, and on the coast line or amid breakers, where these creatures often serve to mark the course of the mariner.
The number of these jelly fishes, often very minute, is beyond calculation or expression, especially in northern waters, where they form the food both of the small crustaceans and other animals upon which the right whales feed, which also devour the jelly fishes in immense numbers.
Adult Sun Fish (Aurelia).
1. Early stage of Jelly Fish (Aurelia). 2. Strobila, more advanced stage. 8. Strobila, ready to be detached, and form the adult (Ephyra).