THE SPONGES (Porifera, or pore-bearers) are among the simplest of animals, being little more than an aggregation of cells. Yet they have gone a step beyond the Protozoa, for there are inner and outer layers of cells, with some hint of specialization of function—some for drawing in water, some for taking in food, some for reproduction. All these cells cohere and work together to carry out the single purpose of the sponge—to pass the waters of the sea through the sieves of its own being. A sponge is an elaborate system of canals contained in a matrix of fibrous or mineral substance, the whole pierced by numerous small entrance pores and larger exit holes. The inmost or central cavities are lined with flagellated cells that remind one of protozoan flagellates. The lashing of the whiplike flagella creates currents to draw in water. In passage through the sponge, the water gives up food, minerals, and oxygen, and carries away waste products.
To a certain extent, each of the smaller groups within the sponge phylum has a physical appearance and habit of life that is characteristic, yet the sponges are probably more plastic in relation to their environment than any other animals. In surf they take the form of a flattened crust, almost without regard to species; in deep, quiet water they may assume an upright tubular form, or branch in a way suggestive of shrubbery. Their shape, therefore, is little or no aid in identification, and the classification of sponges is based chiefly on the nature of their skeleton, which is a loose network of minute hard structures called spicules. In some the spicules are calcareous. In others they are siliceous, although sea water contains only a trace of silica and the sponge must have to filter prodigious quantities to obtain enough for its spicules. The function of extracting silica from sea water is confined to primitive forms of life, and among animals does not occur above the sponges. Commercial sponges fall into a third group, having a skeleton of horny fibers. They are confined to tropical waters.
From such a beginning toward specialization, nature seems to have gone back and made a fresh start with, other materials. All evidence points toward a separate origin for the coelenterates and all other more complex animals, leaving the sponges in an evolutionary blind alley.
THE COELENTERATES, despite their simplicity, foreshadow the basic plan on which, with elaborations, all the more highly developed animals are formed. They possess two distinct layers of cells, the outer ectoderm and the inner endoderm, sometimes with an undifferentiated middle layer that is not cellular but is the forerunner of the third cell layer, the mesoderm, of the higher groups. Each coelenterate is basically a hollow double-walled tube, closed at one end and open at the other. Variations of this plan have resulted in such diverse forms as the sea anemones, corals, jellyfish, and hydroids.
All coelenterates possess stinging cells called nematocysts, each of which is a coiled, pointed thread contained in a sac of turgid fluid, ready to be expelled to impale or entangle passing prey. Stinging cells are not developed in higher animals; although they have been reported in flatworms and sea slugs, they have been secondarily acquired by eating coelenterates.
The Hydrozoa display most clearly another peculiarity of this group, known as alternation of generations. An attached, plantlike generation produces a medusoid generation, shaped like small jellyfish. These, in turn, produce another plantlike generation. In the hydroids the more conspicuous generation is an attached, branching colony bearing tentacled individuals, or hydranths, on its “stems.” Most of these are shaped like small sea anemones and capture food. Other individuals bud off the new generation—tiny medusae that (in many forms) swim away, mature, and shed eggs or sperm cells into the sea. An egg produced by such a medusa, when fertilized, develops into another plantlike stage.
In another group, the Scyphozoa, or true jellyfish, the plantlike generation is the inconspicuous one, and the medusae are highly developed. The jellyfish range from very small creatures to the immense arctic jelly, Cyanea, which reaches an extreme diameter of 8 feet (1 to 3 feet is more common) with tentacles up to 75 feet long.
In the Anthozoa (flower animals) the medusoid generation has been completely lost. This group includes the anemones, corals, sea fans, and sea whips. The anemone represents the basic plan; all the rest of this group are colonial forms in which the individual, anemone-like polyps are embedded in some sort of matrix, which may be stony, as in the reef-building corals, or, in the sea fans and sea whips, may consist of a horny substance of protein nature, similar to the keratin of vertebrate hair, nails, and scales.
Ctenophora: Comb Jellies
THE ENGLISH WRITER Barbellion once said that a comb jelly in sunlight is the most beautiful thing in the world. Its tissues are almost crystal clear, and as this little ovoid creature twirls in the water it flashes iridescent lights. The ctenophores, or comb jellies, are sometimes mistaken for jellyfish because of their transparency, but there are various structural differences, with the “comb-plates” being characteristic of the phylum. These occur in eight rows on the outer surface. Each plate has a hinged attachment and bears hairlike cilia along its free edge; as the plates flash in succession to propel the animal through the water, the cilia break up the rays of sunlight and produce the characteristic flashing.
Like some of the jellyfish, most ctenophores possess long tentacles. These are equipped not with stinging cells, but with sticky pads that capture prey by entanglement. Ctenophores eat enormous numbers of fish fry and other small animals. They live chiefly in the surface waters.
The ctenophores comprise a small phylum, with less than 100 species. Members of one of their groups have flattened bodies and do not swim, but creep on the ocean floor. Some specialists believe these creeping ctenophores have given rise to the flatworms.
THE FLATWORMS include many parasitic as well as many free-living forms. Leafy thin, the free-living flatworms flow like a living film over rocks or sometimes swim by flapping undulations in a way reminiscent of skates. They have made significant advances in an evolutionary sense. They are the first to possess three primary layers of cells, a characteristic of all higher animals. They also have a bilateral type of symmetry (one side being a mirror image of the other), with a head end that always goes first. They have the simple beginnings of a nervous system and eyes that may be only simple pigment spots or, in some species, well-developed organs with lenses. There is no circulatory system, and perhaps it is because of this that all flatworms have such thin bodies, in which all parts are in easy communication with the exterior, and oxygen and carbon dioxide are easily passed through surface membranes to underlying tissues.
Flatworms are found among seaweeds, on rocks, in tide pools, and lurking in dead mollusk shells. They are usually carnivorous, devouring worms, crustaceans, and mollusks of minute size.
THE RIBBON WORMS have extraordinarily elastic bodies, sometimes round, sometimes flat. One of them, the bootlace worm (Lineus longissimus) of British waters, may attain a length of 90 feet and is the longest of all the invertebrates. The American Cerebratulus of shallow coastal waters often is 20 feet long and about an inch wide. Most, however, are only a few inches long and many are considerably less than an inch. They habitually contract into coils or knots when disturbed.