In this respect—in having only one body cavity—sea-anemones and the coral-polyps and the jelly-fishes and the tiny freshwater polyp or hydra, and the marine compound branching polyps like it—agree with one another and differ from the vast majority of animals, such as worms, sea-urchins, star-fishes, whelks, mussels, crustaceans, insects, spiders and vertebrates (which last include fish, reptiles, birds, and mammals). These all have a second chamber, or body cavity, quite shut off from the digestive cavity and from the direct access of water and food particles. This second distinct chamber is filled with an animal fluid, the lymph, and is called the "Cœlom" (a Greek word meaning a cavity). These higher animals, which possess a cœlom as well as a gut, or digestive cavity, are called "Cœlomata," or "Cœlomocœla," in consequence; whilst the sea-anemones, polyps, and jelly-fish form a lower grade of animals devoid of cœlom, but having the one cavity, or gut, continued into all parts of the body. Hence they are called "Cœlentera," or "Enterocœla," words which mean that the cavity of their bodies (Greek cœl) is made by an extension of the gut, or digestive cavity (Greek enteron). The higher grade of animals—the Cœlomocœla—very usually have a vascular system, or blood-vessels and blood, as well as a cœlom and lymph, and quite independent of it; also some kind of kidneys, or renal excretory tubes. Neither of these are possessed by the sea-anemones and their allies—the Enterocœla—but they have, like higher animals, a nervous system and also large ovaries and spermaries on the walls of their single body cavity, which produce their reproductive germs. These pass to the exterior, usually through the mouth, but sometimes by rupture of the body wall.
All "one-cavity" animals, the Enterocœla or Cœlentera, produce peculiar coiled-up threads in their skin in great quantity—many thousands—often upon special warts or knobs. These coiled-up threads lie each in a microscopic sac; they are very delicate and minute and carry a virulent poison, so that they are "stinging" threads. Excitement of the animal, or mere contact, causes the microscopic sac to burst, and the thread to be violently ejected. The sea-anemones, jelly-fish, and polyps feed on fresh living animals, small fish, shrimps, etc., and catch their prey by the use of these poisonous threads. Some jelly-fish have them big enough to act upon the human skin, and bathers are often badly stung by them. The commonest jelly-fish do not sting, but where they occur a few of the stinging sort are likely to occur also. Even some sea-anemones can sting one's hand with these stinging threads. One sea-anemone (known as "Cerianthus"), occasionally taken in British waters, makes for itself a leathery tube by the felting of its stinging threads, and lines its long burrow in the sand below tidal exposure in this way.
The sea-anemones are very hardy, and they are wonderfully varied and abundant on our coasts. Some sixty years ago a great naturalist, who loved the seashore and its rock-pools enthusiastically, Mr. Philip Henry Gosse, father of Mr. Edmund Gosse, the distinguished man of letters, described our British sea-anemones, and gave beautiful coloured pictures of them. One of these I have taken for the frontispiece of this volume, and some of the outline figures of marine animals in these chapters are borrowed from a marvelously complete and valuable little book by him—now long out of print—entitled "Marine Zoology." His books—of high scientific value—and his example, made sea-anemones "fashionable." London ladies kept marine aquariums in their drawing-rooms stocked with these beautiful flowers of the sea. They were exhibited in quantity at the Zoological Gardens in Regent's Park, and it is by no means a creditable thing to our London zoologists that neither these nor other marine creatures are now to be seen there. At a later date public marine aquaria were started with success in many seaside towns,—Brighton, Scarborough, Southport, etc.—and a very fine one was organized in Westminster and another at the Crystal Palace. It is an interesting and important fact, bearing on the psychology of the British people, that most of these charming exhibitions of strange and beautiful creatures from the depths of the sea were very soon neglected and mismanaged by their proprietors; the tanks were emptied or filled with river water, and the halls in which they were placed were re-arranged for the exhibitions of athletes, acrobats, comic singers, and pretty dancers. These exhibitions are often full of human interest and beauty—but I regret the complete disappearance of the fishes and strange submarine animals. I have some hope that before long we may, at any rate in the gardens in the Regent's Park, see really fine marine and fresh-water aquaria established, more beautiful and varied in their contents than those of earlier days.
Fig. 6.—British Sea-Anemones.
a, Sagartia bellis, the daisy anemone, viewed from above when fully expanded.
b, Bunodes crassicornis, half expanded; side view.
c, Anthea cereus. The tentacles are pale apple-green in colour, tipped with mauve, and cannot be completely retracted.
d, Actinia mesembryanthemum. The disk of tentacles is completely retracted. This is the commonest sea-anemone on our South Coast, and is usually maroon colour, but often is spotted like a strawberry.
There are four kinds of sea-anemones which are abundant on our coast. They adhere by a disk-like base to the rocks and large stones, and have the power of swelling themselves out with sea-water (as have many soft-bodied creatures of this kind), with all their tentacles expanded. They have, in that condition, the shape of small "Martello" towers, with their adhesive disk below and the mouth-bearing platform above, fringed by tapering fingers; and they can, on the other hand, shrink to a fifth part of their expanded volume, drawing in and concealing their tentacles, which are in some kinds perforated at the tip. One common on the rocks at Shanklin and other parts of our South Coast, but not on the East Coast, has very abundant, long, pale green tentacles, which are tipped with a brilliant peach colour, and it is peculiar in not being able to retract or conceal this beautiful crown of snake-like locks, reminding one of the Gorgon Medusa. It is known as Anthea cereus ([Fig. 6, c]). Many of them are known by the name "Actinia," and the commonest of all ([Fig. 6, d)] is called "Actinia mesembryanthemum," because of its resemblance to a fleshy-leaved flower of that name which grows on garden rockeries—sometimes called the "ice-plant." This one is of a deep maroon colour, rarely more than an inch and a half across the disk. The adhesive disk is often edged with bright blue, and small spherical tentacles, of a bright blue colour, are set at intervals outside the fringe of longer red ones. This anemone lives wonderfully well in a small glass basin or in an aquarium holding a gallon of sea-water, which is kept duly aerated by squirting it daily. One lived in Edinburgh for more than fifty years, in the possession first of Sir John Dalyell, and then of Mr. Peach. She was known as "Granny," and produced many hundreds of young in the course of years. This species is viviparous, the young issuing from the parent's mouth as tiny fully-formed sea-anemones, which immediately fix themselves by their disks to the glass wall of their habitation. Anemones kept thus in small aquaria have to be carefully fed; bits of the sea mussel (of course, uncooked) are the best food for them. This and many other kinds are not absolutely stationary, but can very slowly crawl by means of muscular movements of the adhesive disk. There are kinds of sea-anemones known which spend their lives floating in the ocean; they are thin and flat. Others adhere to the shells of hermit crabs and even to the big claws of some crabs, and profit by the "crumbs" of food let fall by the nippers of their host. A very handsome and large sea-anemone is common on the East Coast, and is known as "crassicornis" (its generic name is Bunodes). When distended it measures as much as 4 inches across ([Fig. 6, b]). I have one at this moment before me, expanded in a bowl of sea-water. The tentacles are pale green or grey, banded with deep red, and the body is blotched with irregular patches of red, green, and orange. It attaches fine pebbles and bits of shell to the surface of the body.